In a college, 60% of the students are girls. 30% of the girls and 45% of the boys stay in the hostels. What per cent of all the students stay in the hostels?

Take 100 students: 60 girls and 40 boys. Girls in the hostels = 30% of 60 = 18; boys in the hostels = 45% of 40 = 18. Together 36 out of 100, so 36%. 39% swaps the two rates (45% of the girls and 30% of the boys). 37.5% is the plain average of 30 and 45, which would only be right if the two groups were equal in size, and 75% simply adds the two rates.