Question

Let f(n) be the sum of the positive integer divisors of n. For how many values of n, where 1<n<25, is f(n) prime?

Answer 1

I don't know how to answer this but maybe this would help?
There are 18 possible values of n.
Hope this helps! :p

Answer 2

f(2) = 1+2 = 3 ✅
f(3) = 1+3 = 4 ❌
f(4) = 1+2+4 = 7 ✅
f(5) = 1+5 = 6 ❌
and so on