Since A is a set of 5 elements andB be a set of 2 elements.n(A×B)=n(A)×n(B)=5×2=10A subset of A×B with 4 elements will contain any 4 elements of set A×Bout of 10 elements. We have to choose any 4 elements out of 10 elements of A×B.This can be done in 10C4=210 ways.The number of subsets of A×B having 4 elements is 210.