# Combination Probability hopefully an easy one

1. Sep 15, 2010

### clockwork9

Combination Probability... hopefully an easy one :)

I'm doing some CS homework dealing with the efficiency of BitTorrent. Specifically, I'm dealing with combinations of clusters..

So, say, you have file with 5 clusters {A,B,C,D,E}, and you want to pick 3 of them, you the have a choice of 10 cluster combinations: (5!)/(2!*3!)=10

Now, here's where I'm stuck.. I need to setup and equation for the probability that one (or more) specific cluster is in the group you selected. Let's say you pick one of the 10 combinations, now.. are the odds that cluster A is in the group that you picked?
What are the odds clusters A and B are in the group you picked?

I've simplified this, the problem I'm actually dealing this: Bob and Anne are sharing a file on bittorrent with N cluster. Bob is sharing N cluster, anne is sharing Q clusters. N>Q What are the odds that Bob is sharing all the clusters of Q?

You help is very appreciated!
Jim

Last edited: Sep 15, 2010
2. Sep 16, 2010

### awkward

Re: Combination Probability... hopefully an easy one :)

This is really easy. If you select 3 objects from the set {A,B,C,D,E}, the probability that A is one of those selected is 3/5.

Here you lost me. If there are N clusters and Bob is sharing N of them, isn't he sharing ALL the clusters?