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Difference between revisions of "2023 AMC 8 Problems/Problem 25"

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==Solution 1==
 
==Solution 1==
  
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<math>1\leq a_1\leq10</math>,
+
<cmath>1\leq a_1\leq10</cmath>,
 +
 
 +
<cmath>13\leq a_2\leq20</cmath>,
 +
 
 +
<cmath>241\leq a_{15}\leq250</cmath>,
  
−
<math>13\leq a_2\leq20</math>,
 
  
−
<math>241\leq a_{15}\leq250</math>,
 
  
 
We can find the possible values of the common difference by finding the numbers which satisfy the conditions. To do this, we find the minimum of the last two–<math>241-20=221</math>, and the maximum–<math>250-13=237</math>. There is a difference of 13 between them, so only <math>17</math> and <math>18</math> work, as <math>17*13=221</math>, so <math>17</math> satisfies <math>221\leq 13x\leq237</math>. The number <math>18</math> is similarly found. <math>19</math>, however, is too much.
 
We can find the possible values of the common difference by finding the numbers which satisfy the conditions. To do this, we find the minimum of the last two–<math>241-20=221</math>, and the maximum–<math>250-13=237</math>. There is a difference of 13 between them, so only <math>17</math> and <math>18</math> work, as <math>17*13=221</math>, so <math>17</math> satisfies <math>221\leq 13x\leq237</math>. The number <math>18</math> is similarly found. <math>19</math>, however, is too much.
 +
 +
  
 
Now, we check with the first and last equations using the same method. We know <math>241-10\leq 14x\leq250-1</math>. Therefore, <math>231\leq 14x\leq249</math>. We test both values we just got, and we can realize that <math>18</math> is too large to satisfy this inequality. On the other hand, we can now find that the difference will be <math>17</math>, which satisfies this inequality.
 
Now, we check with the first and last equations using the same method. We know <math>241-10\leq 14x\leq250-1</math>. Therefore, <math>231\leq 14x\leq249</math>. We test both values we just got, and we can realize that <math>18</math> is too large to satisfy this inequality. On the other hand, we can now find that the difference will be <math>17</math>, which satisfies this inequality.
 +
 +
  
 
The last step is to find the first term. We know that the first term can only be from <math>1</math> to <math>3</math>, since any larger value would render the second inequality invalid. Testing these three, we find that only <math>a_1=3</math> will satisfy all the inequalities. Therefore, <math>a_{14}=13\cdot17+3=224</math>. The sum of the digits is therefore <math>\boxed{\text{(A)} \hspace{0.1 in} 8}</math>
 
The last step is to find the first term. We know that the first term can only be from <math>1</math> to <math>3</math>, since any larger value would render the second inequality invalid. Testing these three, we find that only <math>a_1=3</math> will satisfy all the inequalities. Therefore, <math>a_{14}=13\cdot17+3=224</math>. The sum of the digits is therefore <math>\boxed{\text{(A)} \hspace{0.1 in} 8}</math>

Revision as of 18:24, 24 January 2023

Problem

Fifteen integers $a_1, a_2, a_3, \dots, a_{15}$ are arranged in order on a number line. The integers are equally spaced and have the property that \[1 \le a_1 \le 10, \thickspace 13 \le a_2 \le 20, \thickspace 241 \le a_{15}\le 250.\] What is the sum of digits of $a_{14}$?

\[\textbf{(A) } 8 \qquad \textbf{(B) } 9 \qquad \textbf{(C) } 10 \qquad \textbf{(D) } 11 \qquad \textbf{(E) } ~12\]

Video Solution

https://www.youtube.com/watch?v=5LLl26VI-7Y&list=PLT9bNzqjDoMnk_gSjh66yRuDMdGecx5zI&index=8

Animated Video Solution

https://youtu.be/itDH7AgxYFo

~Star League (https://starleague.us)

Solution 1

\[1\leq a_1\leq10\],

\[13\leq a_2\leq20\],

\[241\leq a_{15}\leq250\],


We can find the possible values of the common difference by finding the numbers which satisfy the conditions. To do this, we find the minimum of the last two–$241-20=221$, and the maximum–$250-13=237$. There is a difference of 13 between them, so only $17$ and $18$ work, as $17*13=221$, so $17$ satisfies $221\leq 13x\leq237$. The number $18$ is similarly found. $19$, however, is too much.


Now, we check with the first and last equations using the same method. We know $241-10\leq 14x\leq250-1$. Therefore, $231\leq 14x\leq249$. We test both values we just got, and we can realize that $18$ is too large to satisfy this inequality. On the other hand, we can now find that the difference will be $17$, which satisfies this inequality.


The last step is to find the first term. We know that the first term can only be from $1$ to $3$, since any larger value would render the second inequality invalid. Testing these three, we find that only $a_1=3$ will satisfy all the inequalities. Therefore, $a_{14}=13\cdot17+3=224$. The sum of the digits is therefore $\boxed{\text{(A)} \hspace{0.1 in} 8}$


~apex304, SohumUttamchandani, wuwang2002, TaeKim, Cxrupptedpat

Solution 2

Let the common difference between consecutive $a_i$ be $d$. Then, since $a_{15} - a_1 = 14d$, we find from the first and last inequalities that $231 \le d \le 249$. As $d$ must be an integer, this means $d = 17$. Plugging this into all of the given inequalities so we may extract information about $a_1$ gives \[1 \le a_1 \le 10, \thickspace 13 \le a_1 + 17 \le 20, \thickspace 241 \le a_1 + 238 \le 250.\] The second inequality tells us that $a_1 \le 3$, while the last inequality tells us $3 \le a_1$, so we must have $a_1 = 3$. Finally, to solve for $a_{14}$, we simply have $a_{14} = a_1 + 13d = 3 + 221 = 224$, so our answer is $\boxed{\textbf{(A)}\ 8}$. ~eibc

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