3 persons go to a shop for buying a watch.

The price of the watch is 30$ and everybody pays 10$.

After they leave the shop, the seller says to his prentice that the price of the watch was 25$.

He gives 5$ to his prentice and says to him gives back this 5$ to them.

The prentice takes 2$ for himself and gives back 3$ to them.

Now, each person gives 1$, namely each one has paid 9$, that means 9*3=27.

This amount "27$" + "2$" that prentice has is 29$!

Then, where is the other 1$?!