Question

Of the 36 students in the environmental club, 15 or boys and 21 are girls. The president of the club wants to add more boys so that the boys represent 50% of the students in the club.

Answer 1

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.