I know it's something about vector of fractions,but don't know how.
Could anyone explain the solution please?

1) We can only make fractions of form a / 2^b where a is odd (this is obvious, you can prove it by induction for example)

2) If all fractions have this form then we can simply build full binary tree. Number of leaves will be equal to the largest denominator and creatures in leaves can be determined quite easily.

In fact, problem is easy because we aren't asked to find minimal answer.