The mean mark is 70% there are 30 students 20 of which are boys with a mean mark if 62% what is the mean for girls

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