Let n pq be a product of two odd primes p and q how many

Question

This question concerns the quadratic residues in the additive group Z_N .
(An element y ∈ ZN is a quadratic residue if and only if there exists an x ∈ Z_N with 2x = y mod N.)

(a) Let p be an odd prime. How many elements of Z_p are quadratic residues?

(b) Let N = pq be a product of two odd primes p and q. How many elements of _ZN are quadratic residues?

(c) Let N be an even integer. How many elements of Z_N are quadratic residues?