Set of positive integers

Question

Let N be the set of positive integers and, for each n ? N,

An = {x: x is a multiple of n}

(a) (5) Find r such that Ar = A3 A5 .

(b) (5) Find r such that Ar = A4 A6 .

(c) (5) Given that P is the set of prime numbers, describe the set B = U (Ai: i ? P).