Thats wrong because you are adding the remaining balances, for example if u have 10 birr and buy something with 1 birr,9birr left, then if u buy another thing with 1birr ,8 birr left, now what u did is add 9 and 8 which is 17, u dont add those, u add the remaining and the money u spent, which is 8 and 2 birr =10 birr