Question

There are 121pupil in a computer class 8/9 of the boys and 4/5 of the girls were African . The rest were 104 Africans pupils ,how many Africans boy were in the computer class ?

Answer 1

I think you have a typo and you must mean that there were 104 African pupils, then

total number of boys --- x
total number of girls --- 121-x

(8/9)x + (4/5)(121-x) = 104
times 45 to clear fractions
40x + 36(121-x) =4680
40x + 4356 - 36x = 4680
4x = 324
x = 81 <---- total boys
and 40 total girls

<b>So of the 81 total boys (8/9) of them or 72 were African</b>

(and 32 of the 40 girls were African)
check:
40+81 = 121
72+32 = 104