2019 AMC 10B Problem 14

Attempt Problem 14 of the 2019 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 2019 AMC 10B solutions, or check the answer key.

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14.

The base-ten representation for 19!19! is 121,6T5,100,40M,832,H00,121,6T5,100,40M,832,H00, where T,T, M,M, and HH denote digits that are not given. What is T+M+H?T+M+H?

3 3

8 8

12 12

14 14

17 17

Answer: C
Concepts:divisibilitydigitsfactorial
Difficulty rating: 1610
Solution:

Because 19!19! is divisible by 10001000, its last three digits are zero, so H=0H=0.

Since 19!19! is divisible by 99, its digit sum 33+T+M33+T+M is divisible by 99. Hence T+M3(mod9)T+M\equiv3\pmod9, so T+MT+M is either 33 or 1212.

Divisibility by 1111 says the alternating digit sum TM7T-M-7 is divisible by 1111, so TM7(mod11)T-M\equiv7\pmod{11}. Checking the digit possibilities from these two congruences gives T=4T=4 and M=8M=8. Therefore T+M+H=4+8+0=12T+M+H=4+8+0=12.

Thus, the answer is C .

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