Forty five children attend a preschool. The number of two year olds is one half the number of three year olds. The number of four year olds is the same as the number of three year olds.

9 two year olds, 18 three year olds, 18 four year olds

Step-by-step explanation:

Let's say x is the number of 2 year olds. There are double the number of three year olds (2x). Four year olds is the same as 3 year olds (2x).

x + 2x + 2x = 45

5x=45

x = 9

Double is 18 for 3 and four year olds.