Difference between revisions of "2023 AMC 8 Problems/Problem 16"

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The letters <math>\text{P}, \text{Q},</math> and <math>\text{R}</math> are entered into a <math>20\times20</math> table according to the pattern shown below. How many <math>\text{P}</math>s, <math>\text{Q}</math>s, and <math>\text{R}</math>s will appear in the completed table?  
 
The letters <math>\text{P}, \text{Q},</math> and <math>\text{R}</math> are entered into a <math>20\times20</math> table according to the pattern shown below. How many <math>\text{P}</math>s, <math>\text{Q}</math>s, and <math>\text{R}</math>s will appear in the completed table?  
 
<asy>
 
<asy>
/* Made by MRENTHUSIASM, Edited by Kante314 */
 
 
usepackage("mathdots");
 
usepackage("mathdots");
 
size(5cm);
 
size(5cm);
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<math>\textbf{(E)}~134\text{ Ps, }133\text{ Qs, }133\text{ Rs}</math>
 
<math>\textbf{(E)}~134\text{ Ps, }133\text{ Qs, }133\text{ Rs}</math>
  
== Solution 1 (Logic/Finding Patterns)==
+
== Solution 1 (Finding a Pattern)==
  
In our <math>5 \times 5</math> grid we can see there are <math>8,9</math> and <math>8</math> of the letters <math>\text{P}, \text{Q},</math> and <math>\text{R}</math>’s respectively. We can see our pattern between each is <math>x, x+1,</math> and <math>x</math> for the <math>\text{P}, \text{Q},</math> and <math>\text{R}</math>’s respectively. This such pattern will follow in our bigger example, so we can see that the only answer choice which satisfies this condition is <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.</math>
+
In our <math>5\times5</math> grid, there are <math>8,9</math> and <math>8</math> of the letters <math>\text{P}, \text{Q},</math> and <math>\text{R}</math>, respectively, and in a <math>2\times2</math> grid, there are <math>1,2</math> and <math>1</math> of the letters <math>\text{P}, \text{Q},</math> and <math>\text{R}</math>, respectively. We see that in both grids, there are <math>x, x+1,</math> and <math>x</math> of the <math>\text{P}, \text{Q},</math> and <math>\text{R}</math>, respectively. This is because in any <math>n\times n</math> grid with <math>n\equiv2\pmod3</math>, there are <math>x, x+1,</math> and <math>x</math> of the <math>\text{P}, \text{Q},</math> and <math>\text{R}</math>, respectively. We can see that the only answer choice which satisfies this condition is <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.</math>
  
(Note: you could also "cheese" this problem by brute force/listing out all of the letters horizontally in a single line and looking at the repeating pattern. Refer to solution 4)
+
~CoOlPoTaToEs, apex304, SohumUttamchandani, wuwang2002, TaeKim, Cxrupptedpat, Nivaar
 
 
~apex304, SohumUttamchandani, wuwang2002, TaeKim, Cxrupptedpat, Nivaar
 
  
 
== Solution 2 ==
 
== Solution 2 ==
  
We think about which letter is in the diagonal with <math>20</math> of a letter. We find that it is <math>2(2 + 5 + 8 + 11 + 14 + 17) + 20 = 134.</math> The rest of the grid with the <math>\text{P}</math>'s and <math>\text{R}</math>'s is symmetric. Therefore, the answer is <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.</math>
+
Since <math>20\equiv2\pmod3</math> and <math>\text{Q}</math> is in the 2nd diagonal, it is also in the 20th diagonal, and so we find that there are <math>2(2 + 5 + 8 + 11 + 14 + 17) + 20 = 134 \text{Qs}</math>. Since all the <math>\text{P}</math>'s and <math>\text{R}</math>'s are symmetric, the answer is <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.</math>
  
 
~[[User:ILoveMath31415926535|ILoveMath31415926535]]
 
~[[User:ILoveMath31415926535|ILoveMath31415926535]]
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~mathboy100
 
~mathboy100
  
== Solution 4 (Brute-Force) ==
+
== Solution 4 (Brute Force) ==
  
From the full diagram below, the answer is <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.</math>
 
 
<asy>
 
<asy>
/* Made by MRENTHUSIASM, Edited by Kante314 */
 
 
usepackage("mathdots");
 
usepackage("mathdots");
 
size(16.666cm);
 
size(16.666cm);
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drawDiagonal("R", (19.5,19.5));
 
drawDiagonal("R", (19.5,19.5));
 
</asy>
 
</asy>
<b>This solution is extremely time-consuming and error-prone. Please do not try it in a real competition unless you have no other options.</b>
+
 
 +
From the full diagram, the answer is <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.</math>
 +
<b>This solution is extremely time-consuming and error-prone. Do not try it in a real competition unless you have no other options.</b>
  
 
~MRENTHUSIASM
 
~MRENTHUSIASM
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== Solution 6 (Modular Arithmetic)==
 
== Solution 6 (Modular Arithmetic)==
  
Note that the a different letter is always to either the left and right to one letter (there will never be two of the same letter in a row or column).
+
If a letter is in a horizontal position <math>k</math>, then that same letter will appear in position <math>k+3m</math>, for a positive integer <math>m</math>. In other words, all positions congruent to <math>k</math> modulo <math>3</math> will have the same letter as <math>p</math>.  
 
 
It follows that if a letter is in a horizontal position <math>k</math>, then that same letter will appear in position <math>k+3m</math>, for a positive integer <math>m</math>. In other words, all positions congruent to <math>k</math> modulo <math>3</math> will have the same letter as <math>p</math>.  
 
  
 
Since <math>p</math> is in position <math>1</math>, <math>p</math> will be in every position congruent to <math>1 \pmod 3</math>. There are <math>7</math> numbers less than or equal to <math>20</math> that satisfy this restraint. There are also <math>7</math> numbers less than or equal to <math>2</math> that are congruent to <math>2 \pmod 3</math>, but only <math>6</math> that are multiples of <math>3</math>.  
 
Since <math>p</math> is in position <math>1</math>, <math>p</math> will be in every position congruent to <math>1 \pmod 3</math>. There are <math>7</math> numbers less than or equal to <math>20</math> that satisfy this restraint. There are also <math>7</math> numbers less than or equal to <math>2</math> that are congruent to <math>2 \pmod 3</math>, but only <math>6</math> that are multiples of <math>3</math>.  
  
In <math>p</math>'s case, it will appear <math>7</math> times in row one, only <math>6</math> in row <math>2</math> (as its first appearence is in position <math>3</math>), and <math>7</math> in row <math>3</math>. So in the first <math>3</math> rows, <math>P</math> appears <math>20</math> times.  
+
In <math>p</math>'s case, it will appear <math>7</math> times in row one, only <math>6</math> in row <math>2</math> (as its first appearance is in position <math>3</math>), and <math>7</math> in row <math>3</math>. So in the first <math>3</math> rows, <math>P</math> appears <math>20</math> times.  
  
 
Therefore, in the first <math>18</math> rows, <math>P</math> appears <math>20\cdot 6 = 120</math> times. Row <math>19</math> looks identical to row <math>1</math>, as <math>19\equiv 1\pmod 3</math>, so <math>P</math> appears in row <math>19</math> <math>7</math> times. It follows that <math>P</math> appears in row <math>20</math> <math>6</math> times. There are <math>134 P</math>s.  
 
Therefore, in the first <math>18</math> rows, <math>P</math> appears <math>20\cdot 6 = 120</math> times. Row <math>19</math> looks identical to row <math>1</math>, as <math>19\equiv 1\pmod 3</math>, so <math>P</math> appears in row <math>19</math> <math>7</math> times. It follows that <math>P</math> appears in row <math>20</math> <math>6</math> times. There are <math>134 P</math>s.  
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~andyluo
 
~andyluo
  
==Solution 8 (Patterns)==
+
==Solution 8 (Intuitive, Fastest)==
NOTE THAT THIS SOLUTION IS VERY SIMILAR TO SOLUTION 1.
 
 
 
We can start by writing down the <math>P</math>s, <math>Q</math>s, and <math>R</math>s for <math>5\times5, 4\times4, 3\times3,</math> and <math>2\times2</math> squares:
 
 
 
* <math>5\times5: \ 8,9,8</math>
 
* <math>4\times4: \ 6,5,5</math>
 
* <math>3\times3: \ 3,3,3</math>
 
* <math>2\times2: \ 1,2,1</math>
 
 
 
We know that all the squares with side lengths that are multiples of <math>3</math> will have an equal number of <math>P</math>s, <math>Q</math>s, and <math>R</math>s, so we don't worry too much about the <math>3\times3.</math> From the other ones, we can easily see that they are all <math>x, x+1,</math> and <math>x.</math> Using this, we find the only option that provides this format, which is <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.</math>
 
 
 
~CoOlPoTaToEs
 
 
 
==Solution 9 (Intuitive, Fastest)==
 
 
When we find the letters at the corners, we see that there are 2 other corners with Q's and 1 with R. There are 2 corners with Q and 1 with P and R. Therefore, it makes sense that there should be more Q's than other letters. Only <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}</math> has more Q's than other letters.
 
When we find the letters at the corners, we see that there are 2 other corners with Q's and 1 with R. There are 2 corners with Q and 1 with P and R. Therefore, it makes sense that there should be more Q's than other letters. Only <math>\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}</math> has more Q's than other letters.
  
>>> (Only do it this way if there are 30 seconds left on the clock, as it may not always work!)
+
<b>Only do it this way if there are 30 seconds left on the clock, as it may not always work!</b>
  
==Solution 10 (Think and Label)==
+
==Solution 9 (Think and Label)==
Since there are <math>3</math> letters, there must be a multiple of 3 for the height of the array for the amount of letters to be the same. However, there is one less (<math>21-1=20</math>) so there will be one less letter in each column compared to the other two. Looking at the diagram, the first column starts with a <math>P</math> then a <math>Q</math>. Since it will miss the third letter, <math>R</math>, we write this as <math>-R</math>. We do this process for the remaining 20 columns, and applying the same logic if the first three are <math>-R</math>, <math>-P</math> and <math>-Q</math>, there will be one less <math>-Q</math>. This means there will be an extra <math>Q</math>, and scanning the answer choices we find the answer:
+
Since there are <math>3</math> letters, there must be a multiple of 3 for the height of the array for the amount of letters to be the same. However, there is one less (<math>21-1=20</math>) so there will be one less letter in each column compared to the other two. Looking at the diagram, the first column starts with a <math>P</math> then a <math>Q</math>. Since it will miss the third letter, <math>R</math>, we write this as <math>-R</math>. We do this process for the remaining 20 columns, and applying the same logic if the first three are <math>-R</math>, <math>-P</math> and <math>-Q</math>, there will be one less <math>-Q</math> meaning that there will be an extra <math>Q</math>. The only answer that has in extra <math>Q</math> is <math>\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}</math>
<math>\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}</math>
 
  
 
~Blue_Kite
 
~Blue_Kite
  
==Video Solution by Math-X==
+
== Video Solution 1 by Math-X ==
 +
 
 
https://youtu.be/Ku_c1YHnLt0?si=Xxe3CXtrXQYGSYik&t=3081
 
https://youtu.be/Ku_c1YHnLt0?si=Xxe3CXtrXQYGSYik&t=3081
  
~Math-X
+
== Video Solution 2 by CoolMathProblems ==
  
==Video Solution by CoolMathProblems==
 
 
https://youtu.be/_huZfhiCBN8
 
https://youtu.be/_huZfhiCBN8
  
==Video Solution (A Clever Explanation You’ll Get Instantly)==
+
== Video Solution 3 by hnsacademy ==
 +
 
 
https://youtu.be/zntZrtsnyxc?si=nM5eWOwNU6HRdleZ&t=1453
 
https://youtu.be/zntZrtsnyxc?si=nM5eWOwNU6HRdleZ&t=1453
~hsnacademy
 
  
==Video Solution (CREATIVE THINKING!!!)==
+
== Video Solution 4 ==
 +
 
 
https://youtu.be/3DTwjLe0Pw0
 
https://youtu.be/3DTwjLe0Pw0
  
 
~Education, the Study of Everything
 
~Education, the Study of Everything
  
==Video Solution (Animated)==
+
== Video Solution 5 ==
 +
 
 
https://youtu.be/1tnMR0lNEFY
 
https://youtu.be/1tnMR0lNEFY
  
~Star League (https://starleague.us)
+
~[//starleague.us Star League]
 +
 
 +
== Video Solution 6 by OmegaLearn (Using Cyclic Patterns) ==
  
==Video Solution by OmegaLearn (Using Cyclic Patterns)==
 
 
https://youtu.be/83FnFhe4QgQ
 
https://youtu.be/83FnFhe4QgQ
  
==Video Solution by Magic Square==
+
== Video Solution 7 by Magic Square ==
 +
 
 
https://youtu.be/-N46BeEKaCQ?t=3990
 
https://youtu.be/-N46BeEKaCQ?t=3990
==Video Solution by Interstigation==
+
 
 +
== Video Solution 8 by Interstigation ==
 +
 
 
https://youtu.be/DBqko2xATxs&t=1845
 
https://youtu.be/DBqko2xATxs&t=1845
  
==Video Solution by WhyMath==
+
== Video Solution 9 by WhyMath ==
 +
 
 
https://youtu.be/iMhqlz0-ce0
 
https://youtu.be/iMhqlz0-ce0
  
~savannahsolver
+
== Video Solution 10 ==
  
==Video Solution by harungurcan==
 
 
https://www.youtube.com/watch?v=jtHe6yLBz-4&t=20s
 
https://www.youtube.com/watch?v=jtHe6yLBz-4&t=20s
  
~harungurcan
+
== Video Solution 11 by Dr. David ==
  
==Video Solution by Dr. David==
 
 
https://youtu.be/yeXuFQHYU7k
 
https://youtu.be/yeXuFQHYU7k
  
==See Also==  
+
==See Also==
 +
 
 
{{AMC8 box|year=2023|num-b=15|num-a=17}}
 
{{AMC8 box|year=2023|num-b=15|num-a=17}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Revision as of 10:13, 19 January 2025

Problem

The letters $\text{P}, \text{Q},$ and $\text{R}$ are entered into a $20\times20$ table according to the pattern shown below. How many $\text{P}$s, $\text{Q}$s, and $\text{R}$s will appear in the completed table? [asy] usepackage("mathdots"); size(5cm); draw((0,0)--(6,0),linewidth(1.5)+mediumgray); draw((0,1)--(6,1),linewidth(1.5)+mediumgray); draw((0,2)--(6,2),linewidth(1.5)+mediumgray); draw((0,3)--(6,3),linewidth(1.5)+mediumgray); draw((0,4)--(6,4),linewidth(1.5)+mediumgray); draw((0,5)--(6,5),linewidth(1.5)+mediumgray);  draw((0,0)--(0,6),linewidth(1.5)+mediumgray); draw((1,0)--(1,6),linewidth(1.5)+mediumgray); draw((2,0)--(2,6),linewidth(1.5)+mediumgray); draw((3,0)--(3,6),linewidth(1.5)+mediumgray); draw((4,0)--(4,6),linewidth(1.5)+mediumgray); draw((5,0)--(5,6),linewidth(1.5)+mediumgray);  label(scale(.9)*"\textsf{P}", (.5,.5)); label(scale(.9)*"\textsf{Q}", (.5,1.5)); label(scale(.9)*"\textsf{R}", (.5,2.5)); label(scale(.9)*"\textsf{P}", (.5,3.5)); label(scale(.9)*"\textsf{Q}", (.5,4.5)); label("$\vdots$", (.5,5.6));  label(scale(.9)*"\textsf{Q}", (1.5,.5)); label(scale(.9)*"\textsf{R}", (1.5,1.5)); label(scale(.9)*"\textsf{P}", (1.5,2.5)); label(scale(.9)*"\textsf{Q}", (1.5,3.5)); label(scale(.9)*"\textsf{R}", (1.5,4.5)); label("$\vdots$", (1.5,5.6));  label(scale(.9)*"\textsf{R}", (2.5,.5)); label(scale(.9)*"\textsf{P}", (2.5,1.5)); label(scale(.9)*"\textsf{Q}", (2.5,2.5)); label(scale(.9)*"\textsf{R}", (2.5,3.5)); label(scale(.9)*"\textsf{P}", (2.5,4.5)); label("$\vdots$", (2.5,5.6));  label(scale(.9)*"\textsf{P}", (3.5,.5)); label(scale(.9)*"\textsf{Q}", (3.5,1.5)); label(scale(.9)*"\textsf{R}", (3.5,2.5)); label(scale(.9)*"\textsf{P}", (3.5,3.5)); label(scale(.9)*"\textsf{Q}", (3.5,4.5)); label("$\vdots$", (3.5,5.6));  label(scale(.9)*"\textsf{Q}", (4.5,.5)); label(scale(.9)*"\textsf{R}", (4.5,1.5)); label(scale(.9)*"\textsf{P}", (4.5,2.5)); label(scale(.9)*"\textsf{Q}", (4.5,3.5)); label(scale(.9)*"\textsf{R}", (4.5,4.5)); label("$\vdots$", (4.5,5.6));  label(scale(.9)*"$\dots$", (5.5,.5)); label(scale(.9)*"$\dots$", (5.5,1.5)); label(scale(.9)*"$\dots$", (5.5,2.5)); label(scale(.9)*"$\dots$", (5.5,3.5)); label(scale(.9)*"$\dots$", (5.5,4.5)); label(scale(.9)*"$\iddots$", (5.5,5.6)); [/asy] $\textbf{(A)}~132\text{ Ps, }134\text{ Qs, }134\text{ Rs}$

$\textbf{(B)}~133\text{ Ps, }133\text{ Qs, }134\text{ Rs}$

$\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}$

$\textbf{(D)}~134\text{ Ps, }132\text{ Qs, }134\text{ Rs}$

$\textbf{(E)}~134\text{ Ps, }133\text{ Qs, }133\text{ Rs}$

Solution 1 (Finding a Pattern)

In our $5\times5$ grid, there are $8,9$ and $8$ of the letters $\text{P}, \text{Q},$ and $\text{R}$, respectively, and in a $2\times2$ grid, there are $1,2$ and $1$ of the letters $\text{P}, \text{Q},$ and $\text{R}$, respectively. We see that in both grids, there are $x, x+1,$ and $x$ of the $\text{P}, \text{Q},$ and $\text{R}$, respectively. This is because in any $n\times n$ grid with $n\equiv2\pmod3$, there are $x, x+1,$ and $x$ of the $\text{P}, \text{Q},$ and $\text{R}$, respectively. We can see that the only answer choice which satisfies this condition is $\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.$

~CoOlPoTaToEs, apex304, SohumUttamchandani, wuwang2002, TaeKim, Cxrupptedpat, Nivaar

Solution 2

Since $20\equiv2\pmod3$ and $\text{Q}$ is in the 2nd diagonal, it is also in the 20th diagonal, and so we find that there are $2(2 + 5 + 8 + 11 + 14 + 17) + 20 = 134 \text{Qs}$. Since all the $\text{P}$'s and $\text{R}$'s are symmetric, the answer is $\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.$

~ILoveMath31415926535

Solution 3

Notice that rows $x$ and $x+3$ are the same, for any $1 \leq x \leq 17.$ Additionally, rows $1, 2,$ and $3$ collectively contain the same number of $\text{P}$s, $\text{Q}$s, and $\text{R}$s, because the letters are just substituted for one another. Therefore, the number of $\text{P}$s, $\text{Q}$s, and $\text{R}$s in the first $18$ rows is $120$. The first row has $7$ $\text{P}$s, $7$ $\text{Q}$s, and $6$ $\text{R}$s, and the second row has $6$ $\text{P}$s, $7$ $\text{Q}$s, and $7$ $\text{R}$s. Adding these up, we obtain $\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}$.

~mathboy100

Solution 4 (Brute Force)

[asy] usepackage("mathdots"); size(16.666cm);  for (int y = 0; y<=20; ++y) { 	for (int x = 0; x<=20; ++x) {    		draw((x,0)--(x,20),linewidth(1.5)+mediumgray); 		draw((0,y)--(20,y),linewidth(1.5)+mediumgray); 	} }  void drawDiagonal(string s, pair p) { 	while (p.x >= 0 && p.x < 20 && p.y >= 0 && p.y < 20) {     	label(scale(.9)*("\textsf{" + s + "}"),p);         p += (1,-1);     } }  drawDiagonal("P", (0.5,0.5)); drawDiagonal("Q", (0.5,1.5)); drawDiagonal("R", (0.5,2.5)); drawDiagonal("P", (0.5,3.5)); drawDiagonal("Q", (0.5,4.5)); drawDiagonal("R", (0.5,5.5)); drawDiagonal("P", (0.5,6.5)); drawDiagonal("Q", (0.5,7.5)); drawDiagonal("R", (0.5,8.5));  drawDiagonal("P", (0.5,9.5)); drawDiagonal("Q", (0.5,10.5)); drawDiagonal("R", (0.5,11.5)); drawDiagonal("P", (0.5,12.5)); drawDiagonal("Q", (0.5,13.5)); drawDiagonal("R", (0.5,14.5)); drawDiagonal("P", (0.5,15.5)); drawDiagonal("Q", (0.5,16.5)); drawDiagonal("R", (0.5,17.5)); drawDiagonal("P", (0.5,18.5)); drawDiagonal("Q", (0.5,19.5));  drawDiagonal("R", (1.5,19.5));  drawDiagonal("P", (2.5,19.5)); drawDiagonal("Q", (3.5,19.5)); drawDiagonal("R", (4.5,19.5));  drawDiagonal("P", (5.5,19.5)); drawDiagonal("Q", (6.5,19.5)); drawDiagonal("R", (7.5,19.5));  drawDiagonal("P", (8.5,19.5)); drawDiagonal("Q", (9.5,19.5)); drawDiagonal("R", (10.5,19.5));  drawDiagonal("P", (11.5,19.5)); drawDiagonal("Q", (12.5,19.5)); drawDiagonal("R", (13.5,19.5));  drawDiagonal("P", (14.5,19.5)); drawDiagonal("Q", (15.5,19.5)); drawDiagonal("R", (16.5,19.5));  drawDiagonal("P", (17.5,19.5)); drawDiagonal("Q", (18.5,19.5)); drawDiagonal("R", (19.5,19.5)); [/asy]

From the full diagram, the answer is $\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.$ This solution is extremely time-consuming and error-prone. Do not try it in a real competition unless you have no other options.

~MRENTHUSIASM

Solution 5

This solution refers to the full diagram in Solution 4.

Note the $\text{Q}$ diagonals are symmetric. The $\text{R}$ and $\text{P}$ diagonals are not symmetric, but are reflections of each other about the $\text{Q}$ diagonals:

  • The upper $\text{Q}$ diagonal of length $2$ is surrounded by a $\text{P}$ diagonal of length $3$ and an $\text{R}$ diagonal of length $1.$
  • The lower $\text{Q}$ diagonal of length $2$ is surrounded by a $\text{P}$ diagonal of length $1$ and an $\text{R}$ diagonal of length $3.$

When looking at a pair of $Q$ diagonals of the same length $x,$ there is a total of $2x$ $\text{R}$s and $\text{P}$s next to these $2$ diagonals.

The main diagonal of $20$ $\text{Q}$s has $19$ $\text{P}$s and $19$ $\text{R}$s next to it. Thus, the total is $x+1$ $\text{Q}$s, $x$ $\text{P}$s, $x$ $\text{R}$s. Therefore, the answer is $\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.$

~ERMSCoach

Solution 6 (Modular Arithmetic)

If a letter is in a horizontal position $k$, then that same letter will appear in position $k+3m$, for a positive integer $m$. In other words, all positions congruent to $k$ modulo $3$ will have the same letter as $p$.

Since $p$ is in position $1$, $p$ will be in every position congruent to $1 \pmod 3$. There are $7$ numbers less than or equal to $20$ that satisfy this restraint. There are also $7$ numbers less than or equal to $2$ that are congruent to $2 \pmod 3$, but only $6$ that are multiples of $3$.

In $p$'s case, it will appear $7$ times in row one, only $6$ in row $2$ (as its first appearance is in position $3$), and $7$ in row $3$. So in the first $3$ rows, $P$ appears $20$ times.

Therefore, in the first $18$ rows, $P$ appears $20\cdot 6 = 120$ times. Row $19$ looks identical to row $1$, as $19\equiv 1\pmod 3$, so $P$ appears in row $19$ $7$ times. It follows that $P$ appears in row $20$ $6$ times. There are $134 P$s.

Counting $Q$s is nearly identical, but $Q$ begins in position $2$. In the first $18$ rows, there are an identical amount of $Q$s too, namely $6(7+7+6) = 120$. However, by a similar argument to $P$, $Q$ appears $7+7$ times in the last two rows; because row $20$ is the same as row $2$, $Q$ appears in position $1$, and thus $7$ times.

Therefore, there are $120+14 = 134$ $Q$s and $133$ $P$. We could go through a similar argument for $R$, or note that the only answer choice with these two options is $\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.$

-Benedict T (countmath1)

Solution 7 (Answer Choices, Fast)

We can first observe that $P$ shows on diagonals increasing or decreasing by $3.$

It starts at $1,4,7,9...$ and increases in the form $3x-2$. Using our answer choices, $(B)$ and $(C)$ are the only fits.

$Q$ is like this as well, increasing $2,5,8,11....$ This means $Q$ has to be in the form of $3x-1.$ Testing this out leads us with $\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}.$

~andyluo

Solution 8 (Intuitive, Fastest)

When we find the letters at the corners, we see that there are 2 other corners with Q's and 1 with R. There are 2 corners with Q and 1 with P and R. Therefore, it makes sense that there should be more Q's than other letters. Only $\boxed{\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}}$ has more Q's than other letters.

Only do it this way if there are 30 seconds left on the clock, as it may not always work!

Solution 9 (Think and Label)

Since there are $3$ letters, there must be a multiple of 3 for the height of the array for the amount of letters to be the same. However, there is one less ($21-1=20$) so there will be one less letter in each column compared to the other two. Looking at the diagram, the first column starts with a $P$ then a $Q$. Since it will miss the third letter, $R$, we write this as $-R$. We do this process for the remaining 20 columns, and applying the same logic if the first three are $-R$, $-P$ and $-Q$, there will be one less $-Q$ meaning that there will be an extra $Q$. The only answer that has in extra $Q$ is $\textbf{(C)}~133\text{ Ps, }134\text{ Qs, }133\text{ Rs}$

~Blue_Kite

Video Solution 1 by Math-X

https://youtu.be/Ku_c1YHnLt0?si=Xxe3CXtrXQYGSYik&t=3081

Video Solution 2 by CoolMathProblems

https://youtu.be/_huZfhiCBN8

Video Solution 3 by hnsacademy

https://youtu.be/zntZrtsnyxc?si=nM5eWOwNU6HRdleZ&t=1453

Video Solution 4

https://youtu.be/3DTwjLe0Pw0

~Education, the Study of Everything

Video Solution 5

https://youtu.be/1tnMR0lNEFY

~Star League

Video Solution 6 by OmegaLearn (Using Cyclic Patterns)

https://youtu.be/83FnFhe4QgQ

Video Solution 7 by Magic Square

https://youtu.be/-N46BeEKaCQ?t=3990

Video Solution 8 by Interstigation

https://youtu.be/DBqko2xATxs&t=1845

Video Solution 9 by WhyMath

https://youtu.be/iMhqlz0-ce0

Video Solution 10

https://www.youtube.com/watch?v=jtHe6yLBz-4&t=20s

Video Solution 11 by Dr. David

https://youtu.be/yeXuFQHYU7k

See Also

2023 AMC 8 (ProblemsAnswer KeyResources)
Preceded by
Problem 15
Followed by
Problem 17
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All AJHSME/AMC 8 Problems and Solutions

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