Question -
Answer -
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.