Every student of a Mohali school studies Punjabi, Hindi or both, and the diagram gives percentages. What percentage study only Punjabi?

With nobody outside the circles, the three parts make 100%: $?+18+34=100$, so $?=48\%$. 66% is $100-34$ (the whole Punjabi circle), 52% is the whole Hindi circle, and 82% subtracts only the 18%.