20 (boys) the mean was 62% so times that by 20 it will give me 1240 marks.
After that do 30x70 so number of students times the 70% mean which gives 2100.
Now take the 2100 - 1240 = 860.
After that 860/10 = 86%
20 boys get 62 and 10 girls get 86 this all will equal to 2100.....2100/30 = 70
So the final answer is 86%