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HMMT February

Number theory Difficulty 4.8 AIME Find the answer

Find the number of positive divisors dd of 15!=15⋅14⋯2⋅115!=15 \cdot 14 \cdots 2 \cdot 1 such that gcd⁡(d,60)=5\operatorname{gcd}(d, 60)=5.

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

Solution

Since gcd⁡(d,60)=5\operatorname{gcd}(d, 60)=5, we know that d=5id′d=5^{i} d^{\prime} for some integer i>0i>0 and some integer d′d^{\prime} which is relatively prime to 60. Consequently, d′d^{\prime} is a divisor of (15!)/5(15!) / 5; eliminating common factors with 60 gives that d′d^{\prime} is a factor of (72)(11)(13)\left(7^{2}\right)(11)(13), which has (2+1)(1+1)(1+1)=12(2+1)(1+1)(1+1)=12 factors. Finally, ii can be 1,2 , or 3, so there are a total of 3⋅12=363 \cdot 12=36 possibilities.

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