a universal set with n(U)=90

Question

A universal set with n(U)=90 is partitioned into three subsets A,B,C. If n(A)= 3N(B) and n(C)=5n(B), find n(C). 

Answer 1

U contains 90 elements

A+B+C=90

A=3B, C=5B

3B+B+5B=90

9B=90

B=10

A=30

C=5*10=50

n(C)=50