Let x = the number of boys.
Let y = the number of girls.
There are 48 children, therefore
x + y = 48 (1)
There are twice as many boys as girls, therefore
x = 2y (2)
Substitute (2) into (1).
2y + y = 48
3y = 48
y = 48/3 = 16
x = 2y = 2*16 = 32
x=32, y = 16
Answer: 16 girls