There are three groups. One group for girls. One group for half the boys which is equal to one group of girls and another identical sized group of boys because there are twice as many boys as girls.
48=total
3=number of groups
48/3=16
16 is equal to the number of girls because it is equal to half the number of boys
16*2=32
32+16=48
Number of boys plus number number of girls: 32+16=48