Maths Olympiad Prep

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Problem 1059

AMC 12 late, AIME early
Number theory Difficulty 4.7 Find the answer HMMT February · United States

Compute the sum of the positive divisors (including 1) of 9!9! that have units digit 11.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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Official solution

Solution:
The prime factorization of 9!9! is 27⋅34⋅5⋅72^{7} \cdot 3^{4} \cdot 5 \cdot 7. Every divisor of 9!9! has prime factorization 2a⋅3b⋅5c⋅7d2^{a} \cdot 3^{b} \cdot 5^{c} \cdot 7^{d}, where 0≤a≤70 \leq a \leq 7, 0≤b≤40 \leq b \leq 4, 0≤c≤10 \leq c \leq 1, and 0≤d≤10 \leq d \leq 1. If the divisor has units digit 11, it cannot be divisible by 22 or 55, so a=c=0a = c = 0.

Now take cases on the value of dd:
- If d=0d = 0, then the divisor is 3b3^{b} for some 0≤b≤40 \leq b \leq 4. The possible divisors are 11, 33, 99, 2727, and 8181, of which 11 and 8181 work.
- If d=1d = 1, then the divisor is 3b⋅73^{b} \cdot 7 for some 0≤b≤40 \leq b \leq 4. The possible divisors are then 77, 3⋅73 \cdot 7, 9⋅79 \cdot 7, 27⋅727 \cdot 7, and 81⋅781 \cdot 7. Of these, only 3⋅7=213 \cdot 7 = 21 works.

The answer is 1+21+81=1031 + 21 + 81 = \boxed{103}.

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