Cost of Apples

Let see if anyone can figure this out:

A store has 5 apple sales men. The manager gives the first sales man 10 apples, 20 to the second, 30 to the third, 40 to the fourth, 50 to the fifth and 60 apples to the sixth sales man. (i.e. 10 - 20 - 30 - 40 - 50 - 60)

Each sales man has the same conditions of sale, and after they have sold all their apples then all return with 10p.

How come?

Each ones sells a dozen (12) apples for 2p, while robbing others of 1p each for individual apples.