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.
A parallelogram is a quadrilateral with opposite sides parallel. The first figure above is a parallelogram. ... A rectangle is a quadrilateral because it has four sides, and it is a parallelogram because it has two pairs of parallel, congruent sides. All four angles are right angles.
