# Math Help - Combination question

1. ## Combination question

In a poker hand of 5 cards from an ordinary deck of 52 cards, find the probability of getting a full house. Note: a full house is a hand that consists of 3 of one rank and 2 of another.

My solution:

$\frac{13{{4}\choose{3}} * 12{{4}\choose{2}}}{{{52}\choose{5}}}$ $+\frac{12{{4}\choose{3}} * 13{{4}\choose{2}}}{{{52}\choose{5}}}$

But, for some reason my answer is twice the answer given in the solution.

2. ## Re: Combination question

Originally Posted by downthesun01
In a poker hand of 5 cards from an ordinary deck of 52 cards, find the probability of getting a full house. Note: a full house is a hand that consists of 3 of one rank and 2 of another.My solution:
$\frac{13{{4}\choose{3}} * 12{{4}\choose{2}}}{{{52}\choose{5}}}$ $+\frac{12{{4}\choose{3}} * 13{{4}\choose{2}}}{{{52}\choose{5}}}$
But, for some reason my answer is twice the answer given in the solution.
It is because you did the same count twice. WHY?

3. ## Re: Combination question

I realized my mistake while in the car this morning. I multiplied both sides by the permutation of the rank pairs {AAAA},{2222},... instead of multiplying by the combination of rank pairs. My answer should have been:

${{12\choose{2}}*\left(\frac{{{4}\choose{3}} * {{4}\choose{2}}}{{{52}\choose{5}}}$ $+\frac{{{4}\choose{3}} * {{4}\choose{2}}}{{{52}\choose{5}}})$