Alright, lets get started.
Three brothers together borrow $ 72 from their parents.
This $ 72 has an equal share of each brother.
So, share of each brother will be = [tex]\frac{72}{3} = 24[/tex]
So, the same share will be for youngest brother too.
It means youngest brother has to pay $ 24 to his parents.
He already owes $ 15 from his parents.
So, total he must pay back will be = [tex]24 + 15 = 39[/tex]
So, once he returns the money to his parents, his savings will be changed - $ 39 .
So the answer is - $39. : Answer
Hope it will help :)