The lees have three children. the oldest is twice as old as the youngest. the middle child is five years older than the youngest. if the sum of their ages is 57, how old is each child?

Let the youngest be x years. The oldest will be 2x as they are twice the age of the youngest child. The middle child will be x+5 as they are 5 years older than the youngest. So the final equation looks like this:
x + 2x + (x+5) = 57
4x + 5 = 57
4x = 52
x = 13
So the youngest is 13, the oldest is 26 and the middle child is 18. To check, do 13 + 26 + 18 = 57