In the town zoo, 3/28 of the animals are birds. of the birds, 4/15 are birds of prey. what fraction of the animals at the zoo are birds of prey?

For this you have to multiply the two numbers :

are birds and out of those are there relevant birds.
Explanation why you have to multiply:
in a similar situation, if you have a number and you want to find out 3/4 of this number: you multiply the original number by 3/4. Here it's just confusing that both numbers are fractions.

 

so the result is