20 teachers of a school either teach Mathematics or Physics. 12 of them teach Mathematics while 4 teach both the subjects. The number of teachers teaching Physics only, is

8

The correct answer is Option (3) → 8

Let M = Set of teachers teaching Mathematics.

P = Set of teachers teaching Physics.

We have,

$n (M∪P) = 20, n (M) = 12$ and $n (M∩P) = 4$

$∴n(M∪P) =n (M) + n (P)-n (M∩P)$

$⇒20=12+n (P)-4$

$⇒n (P) = 12$

So, the required number = $n (P)-n (M∩P) = 12-4 = 8$