Let X = {1, 2, 3, 4, 5}. The number of different ordered pairs (Y, Z) that can be formed such that
Y⊆ X , Z⊆ X and Y∩Z is empty is
25
53
52
35
For each x ∈ X, we have three choices 5
x ∈ Y, x∉Z , x∈Z ; x∉Y , x∉Z
So the required number of ordered pairs is 35