Grade 9 Math (find missing variable)

y:4.5=13.5:18

?????

set this as two fractions equal to one another
..y.......13.5
------ = -------- cross multiply: top of first x bottom of second = bottom of first x top of second
4.5.......18
18y = 60.75
------ -------- divide by 18 to isolate variable
..18......18
....y = 3.375

13.5*4.5=18*y
so y=(13.5*4.5)/18
y=3.375

y = 13.5 * 4.5 / 18
so y = 3.375

Divide 18 by 4.5