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Difference between revisions of "2001 AIME II Problems/Problem 9"

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(→‎Solution 3 (Case Work))
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Assume the 2-by-2 red square is at <math>C_{11}, C_{12}, C_{21}, C_{22}</math>. To make sure there are no more 2-by-2 red squares, <math>C_{31} and C_{32}</math> can't both be red and <math>C_{13} and C_{23}</math> can't both be red. Meaning that there are <math>2^2-1=3</math> coloring methods for <math>C_{31} and C_{32}</math> and <math>C_{13} and C_{23}</math>. <math>C_{33}</math> can be colored with either colors. However, the coloring method where <math>C_{23}, C_{32}, C_{33}</math> are all red needs to be removed. For exactly one 2-by-2 red square at <math>C_{11}, C_{12}, C_{21}, C_{22}</math>, there are <math>3 \cdot 3 \cdot 2 -1=17</math> coloring methods. As there are <math>4</math> locations for the 2-by-2 red square on the 3-by-3 unit-square grid, if the 3-by-3 unit-square grid has exactly <math>1</math> 2-by-2 red square, there are <math>17 \cdot 4 = 68</math> coloring methods.
 
Assume the 2-by-2 red square is at <math>C_{11}, C_{12}, C_{21}, C_{22}</math>. To make sure there are no more 2-by-2 red squares, <math>C_{31} and C_{32}</math> can't both be red and <math>C_{13} and C_{23}</math> can't both be red. Meaning that there are <math>2^2-1=3</math> coloring methods for <math>C_{31} and C_{32}</math> and <math>C_{13} and C_{23}</math>. <math>C_{33}</math> can be colored with either colors. However, the coloring method where <math>C_{23}, C_{32}, C_{33}</math> are all red needs to be removed. For exactly one 2-by-2 red square at <math>C_{11}, C_{12}, C_{21}, C_{22}</math>, there are <math>3 \cdot 3 \cdot 2 -1=17</math> coloring methods. As there are <math>4</math> locations for the 2-by-2 red square on the 3-by-3 unit-square grid, if the 3-by-3 unit-square grid has exactly <math>1</math> 2-by-2 red square, there are <math>17 \cdot 4 = 68</math> coloring methods.
 +
  
 
Case 2: The 3-by-3 unit-square grid has exactly <math>2</math> 2-by-2 red squares
 
Case 2: The 3-by-3 unit-square grid has exactly <math>2</math> 2-by-2 red squares
  
−
Case 2.1: 2 2-by-2 red squares take up 6 unit grids
+
Case 2.1: <math>2</math> 2-by-2 red squares take up <math>6</math> unit grids
 +
 
 +
Assume the <math>2</math> 2-by-2 red squares are at <math>C_{11}, C_{12}, C_{21}, C_{22}, C_{31}, C_{32}</math>. To make sure there are no more 2-by-2 red squares, <math>C_{13} and C_{23}</math> can't both be red. Meaning that there are <math>2^2-1=3</math> coloring methods for <math>C_{13} and C_{23}</math>. <math>C_{33}</math> can be colored with either colors. However, the coloring method where <math>C_{13}, C_{23}, C_{33}</math> are all red needs to be removed. For exactly <math>2</math> 2-by-2 red squares at <math>C_{11}, C_{12}, C_{21}, C_{22}, C_{31}, C_{32}</math>, there are <math>3 \cdot 2 -1=5</math> coloring methods. As there are <math>4</math> locations for the <math>2</math> 2-by-2 red squares on the 3-by-3 unit-square grid, if the 3-by-3 unit-square grid has exactly <math>2</math> 2-by-2 red squares that take up take up <math>6</math> unit grids, there are <math>5 \cdot 4 = 20</math> coloring methods.
 +
 
 +
Case 2.2: <math>2</math> 2-by-2 red squares take up <math>7</math> unit grids
  
−
Assume the 2-by-2 red square is at <math>C_{11}, C_{12}, C_{21}, C_{22}, C_{31}, C_{32}</math>. To make sure there are no more 2-by-2 red squares, <math>C_{13} and C_{23}</math> can't both be red. Meaning that there are <math>2^2-1=3</math> coloring methods for <math>C_{13} and C_{23}</math>. <math>C_{33}</math> can be colored with either colors. However, the coloring method where <math>C_{13} and C_{23}, C_{33}</math> are all red needs to be removed. For exactly <math>2</math> 2-by-2 red square at <math>C_{11}, C_{12}, C_{21}, C_{22}, C_{31}, C_{32}</math>, there are <math>3 \cdot 2 -1=5</math> coloring methods. As there are <math>4</math> locations for the <math>2</math> 2-by-2 red squares on the 3-by-3 unit-square grid, if the 3-by-3 unit-square grid has exactly <math>2</math> 2-by-2 red squares that take up take up 6 unit grids, there are <math>5 \cdot 4 = 20</math> coloring methods.
+
Assume the <math>2</math> 2-by-2 red squares are at <math>C_{11}, C_{12}, C_{21}, C_{22}, C_{23}, C_{32}, C_{33}</math>. To make sure there are no more 2-by-2 red squares, <math>C_{13}</math> and <math>C_{31}</math> can't be red. Meaning that there is <math>1</math> coloring method for <math>C_{13}</math> and <math>C_{31}</math>. For exactly <math>2</math> 2-by-2 red squares at <math>C_{11}, C_{12}, C_{21}, C_{22}, C_{23}, C_{32}, C_{33}</math>, there is <math>1</math> coloring method. As there are <math>2</math> locations for the <math>2</math> 2-by-2 red squares on the 3-by-3 unit-square grid, if the 3-by-3 unit-square grid has exactly <math>2</math> 2-by-2 red squares that take up take up <math>7</math> unit grids, there are <math>1 \cdot 2 = 2</math> coloring methods.
  
 +
Hence, if the 3-by-3 unit-square grid has exactly <math>2</math> 2-by-2 red squares, there are <math>20+2 = 22</math> coloring methods.
  
  

Revision as of 11:13, 12 April 2023

Problem

Each unit square of a 3-by-3 unit-square grid is to be colored either blue or red. For each square, either color is equally likely to be used. The probability of obtaining a grid that does not have a 2-by-2 red square is $\frac {m}{n}$, where $m$ and $n$ are relatively prime positive integers. Find $m + n$.

Solution 1

We can use complementary counting, counting all of the colorings that have at least one red $2\times 2$ square.

  • For at least one red $2 \times 2$ square:
There are four $2 \times 2$ squares to choose which one will be red. Then there are $2^5$ ways to color the rest of the squares. $4*32=128$
  • For at least two $2 \times 2$ squares:
There are two cases: those with two red squares on one side and those without red squares on one side.
The first case is easy: 4 ways to choose which the side the squares will be on, and $2^3$ ways to color the rest of the squares, so 32 ways to do that. For the second case, there will by only two ways to pick two squares, and $2^2$ ways to color the other squares. $32+8=40$
  • For at least three $2 \times 2$ squares:
Choosing three such squares leaves only one square left, with four places to place it. This is $2 \cdot 4 = 8$ ways.
  • For at least four $2 \times 2$ squares, we clearly only have one way.

By the Principle of Inclusion-Exclusion, there are (alternatively subtracting and adding) $128-40+8-1=95$ ways to have at least one red $2 \times 2$ square.

There are $2^9=512$ ways to paint the $3 \times 3$ square with no restrictions, so there are $512-95=417$ ways to paint the square with the restriction. Therefore, the probability of obtaining a grid that does not have a $2 \times 2$ red square is $\frac{417}{512}$, and $417+512=\boxed{929}$.

Solution 2

We consider how many ways we can have 2*2 grid

$(1)$: All the girds are red--$1$ case

$(2)$: One unit square is blue--The blue lies on the center of the bigger square, makes no 2*2 grid $9-1=8$ cases

$(3)$: Two unit squares are blue--one of the squares lies in the center of the bigger square, makes no 2*2 grid, $8$ cases. Or, two squares lie on second column, first row, second column third row; second row first column, second row third column, 2 extra cases. $\binom 9 2-8-2=26$ cases

$(4)$ Three unit squares are blue. We find that if a 2*2 square is formed, there are 5 extra unit squares can be painted. But cases that three squares in the same column or same row is overcomunted. So in this case, there are $4\cdot (\binom 5 3)-4=36$

$(5)$ Four unit squares are blue, no overcomunted case will be considered. there are $4\cdot \binom 5 4=20$

$(6)$ Five unit squares are blue, $4$ cases in all

Sum up those cases, there are $1+8+26+36+20+4=95$ cases that a 2*2 grid can be formed.

In all, there are $2^9=512$ possible ways to paint the big square, so the answer is $1-\frac{95}{512}=\frac{417}{512}$ leads to $\boxed{929}$

~bluesoul

Solution 3 (Case Work)

\[\begin{array}{|c|c|c|} \hline C_{11} & C_{12} & C_{13}\\ \hline C_{21} & C_{22} & C_{23}\\ \hline C_{31} & C_{32} & C_{33}\\ \hline \end{array}\]

Case 1: The 3-by-3 unit-square grid has exactly $1$ 2-by-2 red square

Assume the 2-by-2 red square is at $C_{11}, C_{12}, C_{21}, C_{22}$. To make sure there are no more 2-by-2 red squares, $C_{31} and C_{32}$ can't both be red and $C_{13} and C_{23}$ can't both be red. Meaning that there are $2^2-1=3$ coloring methods for $C_{31} and C_{32}$ and $C_{13} and C_{23}$. $C_{33}$ can be colored with either colors. However, the coloring method where $C_{23}, C_{32}, C_{33}$ are all red needs to be removed. For exactly one 2-by-2 red square at $C_{11}, C_{12}, C_{21}, C_{22}$, there are $3 \cdot 3 \cdot 2 -1=17$ coloring methods. As there are $4$ locations for the 2-by-2 red square on the 3-by-3 unit-square grid, if the 3-by-3 unit-square grid has exactly $1$ 2-by-2 red square, there are $17 \cdot 4 = 68$ coloring methods.


Case 2: The 3-by-3 unit-square grid has exactly $2$ 2-by-2 red squares

Case 2.1: $2$ 2-by-2 red squares take up $6$ unit grids

Assume the $2$ 2-by-2 red squares are at $C_{11}, C_{12}, C_{21}, C_{22}, C_{31}, C_{32}$. To make sure there are no more 2-by-2 red squares, $C_{13} and C_{23}$ can't both be red. Meaning that there are $2^2-1=3$ coloring methods for $C_{13} and C_{23}$. $C_{33}$ can be colored with either colors. However, the coloring method where $C_{13}, C_{23}, C_{33}$ are all red needs to be removed. For exactly $2$ 2-by-2 red squares at $C_{11}, C_{12}, C_{21}, C_{22}, C_{31}, C_{32}$, there are $3 \cdot 2 -1=5$ coloring methods. As there are $4$ locations for the $2$ 2-by-2 red squares on the 3-by-3 unit-square grid, if the 3-by-3 unit-square grid has exactly $2$ 2-by-2 red squares that take up take up $6$ unit grids, there are $5 \cdot 4 = 20$ coloring methods.

Case 2.2: $2$ 2-by-2 red squares take up $7$ unit grids

Assume the $2$ 2-by-2 red squares are at $C_{11}, C_{12}, C_{21}, C_{22}, C_{23}, C_{32}, C_{33}$. To make sure there are no more 2-by-2 red squares, $C_{13}$ and $C_{31}$ can't be red. Meaning that there is $1$ coloring method for $C_{13}$ and $C_{31}$. For exactly $2$ 2-by-2 red squares at $C_{11}, C_{12}, C_{21}, C_{22}, C_{23}, C_{32}, C_{33}$, there is $1$ coloring method. As there are $2$ locations for the $2$ 2-by-2 red squares on the 3-by-3 unit-square grid, if the 3-by-3 unit-square grid has exactly $2$ 2-by-2 red squares that take up take up $7$ unit grids, there are $1 \cdot 2 = 2$ coloring methods.

Hence, if the 3-by-3 unit-square grid has exactly $2$ 2-by-2 red squares, there are $20+2 = 22$ coloring methods.


~isabelchen

See also

2001 AIME II (Problems • Answer Key • Resources)
Preceded by
Problem 8 		Followed by
Problem 10
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
All AIME Problems and Solutions

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