Poker hand, 2 suits exactly

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SH and adding some number of diamonds and clubs. So every hand is counted in this way, but some are counted more than once. Consider SHD and SHC. The hand made up of the first three spades and the last two diamonds is counted once when you use SHD, but it's counted again when you use SHC. So the number of hands with exactly two suits is less than this number.
  • #1
Hi was hoping someone out there could help me out with this problem.


http://en.wikipedia.org/wiki/Poker_probability

I have looked over the poker probability wiki page, but I cannot figure out this homework problem:


"What is the probability that the hand dealt contains exactly two suits?"

I can justify several different approaches ... but that just means I'm wrong.

1)

(4 C 2) * ( (26 C 5) - 2*(13 C 5) )

ie choose two suits, then from those 26 cards choose 5. Now subtract the two flush hands.


2)

(4 C 2)(26 C 5) - (4 C 1)(13 C 5)

ie get the number of hands with at most two suits then subtract the number of flush hands. (4 C 1)(13 C 5) is the number of flush hands in poker.

3)

(4 C 2)(26 C 5)



JMM
 
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  • #2
hallwayantics said:
Hi was hoping someone out there could help me out with this problem.


http://en.wikipedia.org/wiki/Poker_probability

I have looked over the poker probability wiki page, but I cannot figure out this homework problem:


"What is the probability that the hand dealt contains exactly two suits?"

I can justify several different approaches ... but that just means I'm wrong.

1)

(4 C 2) * ( (26 C 5) - 2*(13 C 5) )

ie choose two suits, then from those 26 cards choose 5. Now subtract the two flush hands.


2)

(4 C 2)(26 C 5) - (4 C 1)(13 C 5)

ie get the number of hands with at most two suits then subtract the number of flush hands. (4 C 1)(13 C 5) is the number of flush hands in poker.

3)

(4 C 2)(26 C 5)



JMM

I'm not sure what you are doing, but shouldn't it go like this:

Looked at in any order--
The probability that a first card will be in one of the four suits is 1.
The probability that a second card will be in a different suit is (51-12)/51.
The probability that the third card will be in one of the two previous suits is (12+12)/50.
Fourth card 23/49.
Fifth card 22/48.

So the answer is 1 * 39/51 * 24/50 * 23/49 * 22/48.
 
  • #3
hallwayantics said:
Hi was hoping someone out there could help me out with this problem. http://en.wikipedia.org/wiki/Poker_probability

I have looked over the poker probability wiki page, but I cannot figure out this homework problem:"What is the probability that the hand dealt contains exactly two suits?"

I can justify several different approaches ... but that just means I'm wrong.

1)

(4 C 2) * ( (26 C 5) - 2*(13 C 5) )

ie choose two suits, then from those 26 cards choose 5. Now subtract the two flush hands.

This one gives the number of hands correctly.

2)

(4 C 2)(26 C 5) - (4 C 1)(13 C 5)

ie get the number of hands with at most two suits then subtract the number of flush hands. (4 C 1)(13 C 5) is the number of flush hands in poker.

This gives the number of hands incorrectly. The problem is that (4 C 2)(26 C 5) doesn't count the number of hands with at most two suits. Thank of it this way. You have 6 ways of picking two suits. You count all the hands made up of just those two suits.

But note, a spade flush gets counted 3 times in this method. It is counted for SD, SC and SH.

So the number hands with exactly two suits is given by the first method, which I calculate as 379236

Method three doesn't try to take proper account of the flush hands, so we'll just pretend it isn't there.

AC130Nav, your method isn't even close. For example, you don't consider cases where the first two cards are the same suit. The original question had the right idea.

Cheers -- sylas

PS. Welcome to physicsforums to you both!
 
  • #4
Thank you, Sylas. The first answer is looking more and more correct to me.

I'm still not quite clear on why the second answer is no good...

It is counted for SD, SC and SH.​

I'm not sure what you mean by this. A flush in spades would mean only spades, why the Diamonds, Clubs, and Hearts?


@AC130

Yeah, that's why I linked to the wiki page, b/c there are different ways to approach probability but I like the nCr way of looking at it.
Thanks for the response.



sylas said:
This one gives the number of hands correctly.



This gives the number of hands incorrectly. The problem is that (4 C 2)(26 C 5) doesn't count the number of hands with at most two suits. Thank of it this way. You have 6 ways of picking two suits. You count all the hands made up of just those two suits.

But note, a spade flush gets counted 3 times in this method. It is counted for SD, SC and SH.

So the number hands with exactly two suits is given by the first method, which I calculate as 379236

Method three doesn't try to take proper account of the flush hands, so we'll just pretend it isn't there.

AC130Nav, your method isn't even close. For example, you don't consider cases where the first two cards are the same suit. The original question had the right idea.

Cheers -- sylas

PS. Welcome to physicsforums to you both!
 
  • #5
hallwayantics said:
Thank you, Sylas. The first answer is looking more and more correct to me.

I'm still not quite clear on why the second answer is no good...

It is counted for SD, SC and SH.​

I'm not sure what you mean by this. A flush in spades would mean only spades, why the Diamonds, Clubs, and Hearts?

The second method considers 6 ways of picking two suits. The suits are SHDC, so the six ways are SH, SD, SC, HD, HC, DC.

Now, given a pair of suits, you count up all the hands made up of those two suits only (and possibly even one of them). So let SH be the set of all hands made up of spades and hearts only.

Any spade flush hand will be counted as a hand in SH, and a hand in SD, and a hand in SC.

Now #SH, of the number of hands in SH, is (26 C 5).

but 6 * #SH is not the number of hands in (SH + SD + SC + HD + HC + DC). Those six sets are not disjoint! They overlap. Any flush hand will appear in three of the six sets.

It follows that #(SH + SD + SC + HD + HC + DC) = 6.#SH - 2.#FLUSH = 6(26 C 5) - 8(13 C 5)

Hence, in method two, where you have (4 C 2)(26 C 5) (for the number of hands with two or less suits) you should have (4 C 2)(26 C 5) - 8(13 C 5)

Now from this count, you remove all the flushes, which is removing another 4(13 C 5).

The final answer becomes (4 C 2)(26 C 5) - 12 (13 C 5)

And that gives the same result as method 1 again.

Cheers -- Sylas
 
Last edited:
  • #6
The two methods which give the right answer for the number of hands are:

(4 C 2) (26 C 5) - 3 (4 C 1) (13 C 5)

and

(4 C 2) ( (26 C 5) - 2 (13 C 5) )

It's easy to show they give the same result.

But how about this method? Any hand having exactly two suits must have more of one suit that the other. It has 3 of this suit, and 2 of the other; or it has 4 of this suit and 1 of the other.

There are 4 ways to pick the suit which appears most often in the hand. There are 3 ways to pick the other suit, which appears less often.

The total number of hands is therefore:

4 * 3 * ( (13 C 3) (13 C 2) + (13 C 4) ( 13 C 1) )

Try it. You should get the same number, although it is by no means obvious just from the expression themselves. Combinatorics is full of cases like this, where the same count can be obtained with two methods that otherwise seem quite unrelated.

Cheers -- sylas
 
  • #7
Yes, it is neat how combinatoric problems can be approached from different angles. I'm enjoying it much more than the regular set theory about inclusion/exclusion intersection/union, proofs etc. Somehow combinatorics makes me feel like I'm doing math back in grade school again (back when I was good at it!)

Thanks again!
sylas said:
The two methods which give the right answer for the number of hands are:

(4 C 2) (26 C 5) - 3 (4 C 1) (13 C 5)

and

(4 C 2) ( (26 C 5) - 2 (13 C 5) )

It's easy to show they give the same result.

But how about this method? Any hand having exactly two suits must have more of one suit that the other. It has 3 of this suit, and 2 of the other; or it has 4 of this suit and 1 of the other.

There are 4 ways to pick the suit which appears most often in the hand. There are 3 ways to pick the other suit, which appears less often.

The total number of hands is therefore:

4 * 3 * ( (13 C 3) (13 C 2) + (13 C 4) ( 13 C 1) )

Try it. You should get the same number, although it is by no means obvious just from the expression themselves. Combinatorics is full of cases like this, where the same count can be obtained with two methods that otherwise seem quite unrelated.

Cheers -- sylas
 

1. What is the probability of getting a poker hand with exactly 2 suits?

The probability of getting a poker hand with exactly 2 suits is approximately 23.5%. This means that out of all possible poker hands, about 23.5% will have exactly 2 suits.

2. How many different combinations of 2 suit poker hands are there?

There are 5,148 different combinations of 2 suit poker hands. This can be calculated by taking the number of ways to choose 2 suits from a deck of 4 suits (4 choose 2), multiplied by the number of ways to choose 5 cards from each of those 2 suits (13 choose 5).

3. What is the highest ranking hand possible with 2 suits?

The highest ranking hand possible with 2 suits is a straight flush. This is when you have 5 cards of the same suit in consecutive order. For example, 8, 9, 10, J, Q of hearts or diamonds.

4. Can a full house be made with 2 suits?

Yes, a full house can be made with 2 suits. A full house is when you have 3 cards of one rank and 2 cards of another rank. In a 2 suit poker hand, this would mean having 3 cards of the same suit and 2 cards of a different suit, or vice versa.

5. Is it more advantageous to have a 2 suit hand compared to a 3 or 4 suit hand?

It is generally not more advantageous to have a 2 suit hand compared to a 3 or 4 suit hand. While a 2 suit hand may have a slightly higher probability of occurring, it does not necessarily guarantee a higher ranking hand. In poker, it is more important to focus on the value and ranking of your individual cards rather than the number of suits in your hand.

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