45 children attend preschool the number of two-year-olds is 1/2 the number of three-year-olds the number for your old is the same as the number of three-year-olds

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.