I'm suppose return a change to user.
Initially, the machine has
Bills - 0
quarters - 5
dimes - 5
nickels - 5
user puts a money to machine(each coins are sum up to only maxium $1 i.e, user can only put 10 dimes maximum, or 20 nickels) but only a dollar to machine.
let's say user buys a candy which cost 65 cents. like in real situation vending machine, if the machine does not have enough quarters, then it should give changes in dimes/nickels.
also, if change in machine does not have any chnage or enough change, it should just return whatever user put in previously.
moreover, for a 65 cents candy, the machine does not have any change left and user put
15 nickels(15 x 5= 75cents) then it should return 2 nickels.