Misal n(S) – n(A) =5 dan 2n(S) – n(A) = 13

N(S) – n(A) = 5
n(A) = 5 – n(S)

2n(S) – A = 13
2S – ( S – 5 ) = 13
2S – S + 5 = 13
S + 5 = 13
n(S) = 13 – 5 = 8
n(S) = 8

nS – nA = 5
8 – A = 5
n(A) = 8 – 5 = 3

Banyak anggota A = 3 (a)