Oldest Child to Youngest Child

Question

How do I put this together to get the answer?

The oldest child in a family of four is twice as old as the youngest. The two middle children are 12 and 15 years old. If the average age of the children is 15, how old is the youngest child?

Answer 1

2y+y+12+15=4(15)=60

3y=60-27=33

y=33/3 =11 years old = youngest age

(11+12+15+22)/4 =60/4=15