2019 AMC 10A Problem 2

Below is the professionally curated solution for Problem 2 of the 2019 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 10A solutions, or check the answer key.

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Concepts:factorialtrailing zerosdivisibility

Difficulty rating: 770

2.

What is the hundreds digit of (20!15!)?(20!-15!)?

00

11

22

44

55

Solution:

Note that 20!20! and 15!15! both have factors of 535^3 in them.

This means that they are both divisible by 1000,1000, making their difference also a multiple of 1000.1000.

Being a multiple of 10001000 makes the last three digits 0,0, which shows that the hundreds digit is also 0.0.

Thus, A is the correct answer.

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