Find the number of positive divisors d of 15!=15⋅14⋯2⋅1 such that 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, we know that d=5id′ for some integer i>0 and some integer d′ which is relatively prime to 60. Consequently, d′ is a divisor of (15!)/5; eliminating common factors with 60 gives that d′ is a factor of (72)(11)(13), which has (2+1)(1+1)(1+1)=12 factors. Finally, i can be 1,2 , or 3, so there are a total of 3⋅12=36 possibilities.
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