how to work this out?

[John D'Errico]

But this is YOUR homework. Not ours. You work it out by starting to write code. MAKE AN EFFORT. We don't do your homework for you. Once you have done some work, if you have a specific question, then ask it.

[Stephen23]

Exact duplicate, which already has an accepted answer:

http://www.mathworks.com/matlabcentral/answers/273994-design-an-algorithm-and-write-the-program-for-a-machine-that-must-give-the-current-amount-of-change

@beepo pony: If you have any question about that method, then make a comment in that thread.