There are 20 balls. If each boy serves two balls, there will be two more. In fact, only 20-2= 18 balls are needed, and each boy can get two.

18 ball, two for each person, one * * has 18/2=9 boys.

If every girl sends two, there will be two less. Actually, 20+2=22 balls are needed, and each girl is assigned two balls, 22/2= 1 1 girl.