Let A be the set of teachers who teach mathematics and B be the set of teachers who teach physics. Given: n(A∪B)=20,n(A)=12,n(A∩B)=4 Here, we have to find how many teachers teach only physics ie n (B - A)As we know that, n(A∪B)=n(A)+n(B)−n(A∩B) ⇒20=12+n(B)−4 ⇒n(B)=12 As we know that, n(B−A)=n(B)−n(A∩B) ⇒n(B−A)=12−4=8 So, there are 8 teachers out of 20 teachers who teach only physics Hence, option B is the correct answer.