Find How Many Teachers Teach Physics
Question:
In a school there are 20 teachers who teach mathematics or physics. Of these, 12 teach mathematics and 4 teach physics and mathematics. How many teach physics?
Solution
\[ n(M\cup P)=n(M)+n(P)-n(M\cap P) \] \[ 20=12+n(P)-4 \] \[ 20=8+n(P) \] \[ n(P)=20-8 \] \[ n(P)=12 \]Hence, 12 teachers teach physics.