If the number of items in a set A is n(A) = 40. If n(B) = 26 and n(A ∩ B) = 16 then n(A ∩ B) is equal to:

As we know the formula.
n(A ∪ B ) = n(A) + n(B) − n(A ∩ B )
Given in the question,
n(A) = 40;
n(B) = 26;
n(A ∩ B ) = 16;
Put these value in formula and solve the equation.
n(A ∪ B ) = n(A) + n(B) − n(A ∩ B )= 40 + 26 – 16 = 50