There are 9 two-year-olds and 18 each of 3 and 4 year olds.
To solve this we could write and solve the equation below, letting x be the number of 3 and 4 year olds.
x/2 + x + x = 45
x + 2x + 2x = 90
5x = 90
x = 18
From here, we just divide 18 by 2 to get the number of 2 year olds.
