A hostel in Dehradun has 90 boys. The diagram shows how many play chess, carrom or both. How many boys play neither game?

Boys who play at least one game $=28+11+36=75$, so neither $=90-75=15$. 4 subtracts both full circles (39 and 47) and so removes the 11 twice, 26 ignores the overlap, and 75 is the players, not the rest.