mathematical

Another way you can look at it to get back to 100; he pocketed one dollar and paid them each one dollar. Since he's adding together how much he continues to owe, he needs to also add how much he's already paid them.

Actually you don't keep the spare dollar. The thing that is wrong is that you are adding the money that you owe($98) and then add the money that you have ($1) together when like 98+1=99 when you are actually supposed to say -98+1=-97 which is the money that you used on she shirt.

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