Answer:
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
