If X and Y are two sets such that X has 40 elements, X ∪ Y has 60 elements and X ∩ Y has 10 elements, how many elements does Y have?

We have n(X) = 40 
n(X ∪ Y) = 60 
n(X ∩ Y) = 10 
By using the formula,
         n(X ∪ Y) = n(X) + n(Y) - n(X ∩ Y)
We have 60 = 40 + n(Y) - 10 
           n(Y) = 60 - 40 + 10 
           n(Y) = 60 - 30
.^..        n(Y) = 30.