In a school, there are 20 teachers who teach mathematics or physics. Of these, 12 teach mathematics

We have,

Teachers teaching physics or math = 20

Teachers teaching physics and math = 4

Teachers teaching maths = 12

Let teachers who teach physics be ‘n (P)’ and for Maths be ‘n (M)’

Now,

20 teachers who teach physics or math = n (P ∪ M) = 20

4 teachers who teach physics and math = n (P ∩ M) = 4

12 teachers who teach maths = n (M) = 12

We know,

n (P ∪ M) = n (M) + n (P) – n (P ∩ M)

Substituting the values we get,

20 = 12 + n (P) – 4

20 = 8 + n (P)

n (P) =12

There are 12 physics teachers.