Maths Olympiad Prep

Library / /101 of 860

Number theory Difficulty 4.8 AIME Find the answer

Find the number of positive divisors dd of 15!=15142115!=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=5idd=5^{i} d^{\prime} for some integer i>0i>0 and some integer dd^{\prime} which is relatively prime to 60. Consequently, dd^{\prime} is a divisor of (15!)/5(15!) / 5; eliminating common factors with 60 gives that dd^{\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 312=363 \cdot 12=36 possibilities.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.