Class 10R sat a test. The mean mark of the class was 70%. There are 30 students in class 10R, 20 of whom are boys. The mean mark for the boys in the test was 62%. Work out the mean mark for the girls.

24%

This problem can be solved by working backwards.

Average of 30 students = 70.

Total marks of all the 30 students = 30x70 = 2100

Out of these 30, average of 20 students = 62

Total marks of these 30 students = 30x62 = 1860

So the total marks of the remaining 10 students = 2100-1860 = 240

Hence the average of the remaining 10 students = 240/10 = 24