When 18 boys joined a group of y students, the ratio of boys to girls in the group became 4: 5. write an algebraic expression for the number of girls in terms of y.

Boys + Girls = y
( B + 18 ) : G = 4 : 5
5 ( B + 18 ) = 4 g
5 B + 90 = 4 G
5 B = 4 G - 90   / : 5
B = 4/5 G - 18
Then:
4/5 G - 18 + G = y
9/5 G = y + 18  / * 5/9
G = 5 * ( y + 18 ) / 9

Number of girls in terms of y is:
G = 5/9 y + 10