In a group of 48 children there are twice as many boys as girls how many girls are they

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

16 girls