In a school there are 20 teachers who teach mathematics or physics. Of these, 12 teach mathematics and 4 teach physics and mathematics. How many teach physics?
Let A be the number of teachers who teach mathematics & B be the number of teachers who teach physics.
$n(A \cup B)=20$
$n(A \cap B)=4$
$n(A \cup B)=n(A)+n(B)-n(A \cap B)$
Therefore, 12 teachers teach physics.