The ratio of ages (in years) of three children is 2:4:5. The sum of their ages is 33. What is the ages of each child?

You could also write it as:

1st child's age: = 2x

2nd child: = 4x

3rd child: = 5x

2x+4x+5x=33

11x=33

x=3

Then

1st child's age: = 2x = 6

2nd child: = 4x = 12

3rd child: = 5x = 15

To check if it's correct you need to sum up: 6+12+15=33

2+4+5 is 11 and 33/11 is 3 so for every 1 is 3 years,

the first is 2 so is 2x3=6

the second is 4 so is 4x3=12

the third is 5 so is 5x3=15