Homework Help: Probability Question

1. Sep 12, 2010

mynameisfunk

1. The problem statement, all variables and given/known data

prove that
$$\sum$$nk=0 ($$^{n}_{k}$$) ($$^{m-n}_{n-k}$$) = ($$^{m}_{n}$$)

2. Relevant equations
2n=$$\sum$$$$^{n}_{k=0}$$ ($$^{n}_{k}$$)

3. The attempt at a solution
I know that every summand on the left hand side is a member of the power set times the combinations of it's complement and we can think of them in terms of the set changing with every possible combination, 1 element at a time and 2 at a time, 3 at a time, ..., n at a time. Or something like that... Definitely having trouble putting this into words let alone an equation. I certainly see the connection but am needing a little help on the proof. Please not the whole proof but just a little help putting it into words?

Last edited: Sep 13, 2010
2. Sep 12, 2010

cronxeh

3. Sep 13, 2010

mynameisfunk

Having some trouble getting my head around this still.