2023 AMC 8 Problem 25

Below is the video solution and professionally curated solution for Problem 25 of the 2023 AMC 8, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2023 AMC 8 solutions, or check the answer key.

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Concepts:arithmetic sequencebounding to limit casesdigits

Difficulty rating: 1950

25.

Fifteen integers a1,a2,a3,,a15a_1, a_2, a_3, \cdots, a_{15} are arranged in order on a number line. The integers are equally spaced and have the property that 1a110, 1 \leq a_1 \leq 10,13a220, 13 \leq a_2 \leq 20, and 241a15250. 241 \leq a_{15} \leq 250.

What is the sum of the digits of a14?a_{14}?

88

99

1010

1111

1212

Video solution:
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Written solution:

Let dd be the common difference. If we let a1=10a_1 = 10 and a15=241,a_{15} = 241, we see that d2411014=16.5. d \geq \dfrac{241 - 10}{14} = 16.5. Since all the numbers are integers, dd must be at least 17.17.

Also, if a1=1a_1 = 1 and a15=250,a_{15} = 250, we get that d25011417.8. d \leq \dfrac{250 - 1}{14} \approx 17.8. Once again since all the numbers are integers, dd is at most 17.17. This tells us that dd is 17.17.

Note that 1714=238.17 \cdot 14 = 238. This means that a1a_1 must be at least 33 for a15a_{15} to be within the desired range.

If a1a_1 is greater than 3,3, however, a2a_2 becomes greater than 20,20, which is not allowed.

Now we know that a1=3a_1 = 3 and d=17.d = 17. This tell us that a14=3+1317=224. a_{14} = 3 + 13 \cdot 17 = 224.

Therefore, sum of the digits is 2+2+4=8.2 + 2 + 4 = 8.

Thus, A is the correct answer.

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