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2019 AMC 10B Problem 14

Problem 14 of 25IntermediateNumber TheoryCounting & Probability

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

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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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Concepts: divisibility · digits · factorial

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.