If S and T are two sets such that S has 21 elements, T has 32 elements, and S ∩T has 11 elements, how many elements does S ∪ T have?

We have n(S) = 21
             n(T) = 32 
       n(S ∩ T) = 11
By using the formula,
      n(S ∪ T) = n(S) + n(T) - n(S ∩ T)
We have n(S ∪ T) = 21 + 32 - 11
.^..   n(S ∪ T) = 53 - 11
.^..   n(S ∪ T) = 42.