I am not getting how to solve this problem:

There are N shoppers today at the Bytelandian Shopping Mall. There are M different items at sale at the mall. In how many ways can shoppers purchase R items in all such that everyone buys atleast one item, and every item is brought by atleast one person ? A shopper can purchase any particular item at most once.

My brother has knowledge of terrible sludge problem , if anybody wants more information contact us.
Motorhome Insurance Quote