# How to determine combinations correctly

1. Apr 20, 2012

### Mathematicize

Hey everyone,

I have a question regarding combinations and poker! Could someone explain to me why the number of distinct ways to receive a full house from a five card draw is:

13 nCr 1 * 4 nCr 3 * 12 nCr 1 * 4 nCr 2
and not,
13 nCr 2 * 4 nCr 3 * 4 nCr 2

I usually make small mistakes here and there with combinatorics and I can never find a good answer from anyone. Also, I am more interested in the not in this case. Intuitively they seem to say the same thing to me. (choose two distinct denominations, choose 3 cards for the first denomination and 2 cards for the second denomination). Also does anyone have a good way at viewing combinations other than the number of subsets of size r from a set of size n?

Thanks!

2. Apr 20, 2012

### chiro

Hey Mathematicize and welcome to the forums.

It might help the readers here if you show your thought process on how you derived your answer because doing this will help us understand where the thinking was wrong or even if the answer that has been provided is wrong if this is the case.

3. Apr 21, 2012

### Mathematicize

The first answer is correct after computing the probabilities and checking with various sources. So I will explain my thought process on the second answer in which I obtained. First I did 13 nCr 2 because in a full house we need 2 distinct denominations where order does not matter. Next, we must pick the suit for the first denomination, and there are 4 suits and 3 that we must choose in which order does not matter, so I did 4 nCr 3. Next I needed 2 suits for the second denomination. So with the same reasoning as my last step I get 4 nCr 2. Then since each step is independent of each other, I invoke the multiplication rule to get the total number of ways to receive a full house.

The different between the answers is off by a factor of 1/2. I have a hard time visualizing this or any real way at checking my work for correctness which is a big problem.

Last edited: Apr 21, 2012
4. Apr 21, 2012

### Stephen Tashi

You have to account for the number of ways that one of the denominations can be designated as "the first" deonomination. When you enumerated picking the denominations, you didn't enumerate it as picking a "first" and "second" denomination.