Andy and Diana both got some pocket money from their aunt.
Andy spent all his money on collector's coins, he bought 5 of them.
Diana bought 3 of such coins, and she brought home 20 dollars of change.
after they came home their sister said "hah! I bet you I can find out how much each coin costs even if you don't tell me how much money your aunt gave you, if she gave you both the same amount!"
So, how much does each coin cost?