In a school there are 20 teachers who teach mathematics or physics.

Let A be the number of teachers who teach mathematics & B be the number of teachers who teach physics.

Given :

$n(A)=12$

$n(A \cup B)=20$

$n(A \cap B)=4$

To find:

$n(B)$

We know:

$n(A \cup B)=n(A)+n(B)-n(A \cap B)$

$\Rightarrow 20=12+n(B)-4$

$\Rightarrow n(B)=20-8=12$

Therefore, 12 teachers teach physics.