2016 AMC 10B Problem 8

Attempt Problem 8 of the 2016 AMC 10B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2016 AMC 10B solutions, or check the answer key.

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8.

What is the tens digit of 20152016−2017?2015^{2016}-2017?

 0 \ 0

 1 \ 1

 3 \ 3

 5 \ 5

 8 \ 8

Answer: A
Concepts:modular arithmeticChinese Remainder Theorem
Difficulty rating: 1420
Small Hint:

Work modulo 100100 to get the tens digit

Big Hint:

Use congruences modulo 44 and 2525

Solution:

To find the tens digit, we first need to find the number modulo 100.100. First, we can find 20152016mod  100.2015^{2016} \mod 100. We can do this with the Chinese Remainder Theorem by first getting the number mod  4\mod 4 and then mod  25.\mod 25.

The number 20152016≡0mod  252015^{2016} \equiv 0 \mod 25 since it is a multiple of 25.25.

Then, observe that 20152016≡(−1)20162015^{2016} \equiv (-1)^{2016}≡1mod  4. \equiv 1 \mod 4 .

By the Chinese remainder theorem, we can get that our number is congruent to 25mod  100.25 \mod 100.

Therefore, 20152016−20172015^{2016}-2017 ≡25−17mod  100\equiv 25 - 17 \mod 100≡8mod  100.\equiv 8 \mod 100 . This means the tens digit is 0.0.

Thus, the correct answer is A .

Problem 7#7
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