Your answer is:
(125 - 8x^3) / (25 + 10x + 4x^2) = - ((2x - 5) * (4x^2 + 10x + 25)) / (4x^2 + 10x + 25) = - (2x - 5) = - 2x + 5
The correct result would be - 2x + 5.
Because 50% of something means half (because 100-50=50), he would have to make the number of girls and boys equal. Since there are 21 girls, he has to make it so there are also 21 boys.
So, if we know that 21-15=6, this means he would have to add 6 boys to the club if he wants the girls and boys equal.
The formula for this is

. If the diameter is 8 the radius is 4, so filling in accordingly we have

and

which is

and when you multiply in 3.14 you get that A = 401.92 or B above.
Well the way I see it is he started with 4 villager so I would see the equation to be
1.13x times 17+4 would equal the amount of villagers he could have which would be 23.21 as for the equation it would be 1.13x times 17 plus four equals the number of villagers total