two girls

I hope my bad english not make any problems with this
Two girls goes to shop to buy shoes. Every girl had a 7,5$. But unfortunately, this shoes already be sold to another peoples Then girls found something with price 15$ and buy it together (7,5 + 7,5 = 15$) and happy go home. After some time shop found that this item price = 10$ not 15$, and give those 5$ to someone who knows this girls. This dude think that girls don't know right price, and give every girl 1$ and 3$ get for yourself. Girls happy think that she pays 6,5$ (7,5$ - 1$) and will be happy Ok, let's think girls pay's 6,5+6,5=13$ and dude have 3$ 13+3=16$!!! From where there is 1$ more?