You need new shoes but you do not have money.
And you borrow to 2 of your friends each 50 $.
When you want to buy shoes, shoes price was only 97$. you restore each 1$ to your friends and the remaining $ 1 you buy a drink.
so, the rest of your debt to your friends remaining 49$.
if 49 + 49 = 98 (because you have to restore 1$) so, if coupled with the money that you traded for a drink only 1$. mean 98 + 1 = 99. where 1$ is?