Combinatorics

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by lunarpower » Thu Jun 16, 2011 4:21 pm
chendawg wrote: So with this in mind, am I doing these scenarios and applying this principle correctly?

If I want to pull a King and Queen for hole cards (suits do not matter, nor does order), then the probability would be 8/663(4/52*4/51*2). I would be multiplying by two since the cards are from the same pool.
correct
If I want to pull a King and Queen(suits don't matter, nor order), with a pull each from TWO standard decks, then it would be 4/169(8/52*8/52). I'm not multiplying by two this time because they are from pulls from separate decks each time.
no, this is wrong.
imagine that the decks are green and blue.
in this case, you must actually add probabilities for both events, because they are qualitatively different events: i.e., drawing a king from the green deck and a queen from the blue deck is not the same thing as drawing a queen from the green deck and a king from the blue one.
so this would be (4/52)(4/52) + (4/52)(4/52), i.e., (green king)(blue queen) + (blue king)(green queen).

what you don't switch is the order of the pools themselves -- in other words, once you've settled on selecting from the green deck first and from the blue deck second, you calculate ALL probabilities using that same selection order.
so, for instance, the probability of getting a king from both decks would just be (4/52)(4/52).
If I want to pull a King and Queen(suits don't matter, nor order), with a pull from TWO standard decks, but this time we combine the decks and pull the cards, then it would be 60/1339(16/104*15/103*2). I would be multiplying by 2 this time since it's from the same pool.
two errors.
first, you appear to be operating under the assumption that each deck contains 8 kings and 8 queens. (weird, because you had the right numbers of kings and queens in the first part.)
second, i don't know why you decreased the numerator; that is what you'd do if you were picking two of the same card.
the numerators should be 8 and 8, not 16 and 15. otherwise, correct.
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by chendawg » Thu Jun 16, 2011 4:48 pm
lunarpower wrote:
If I want to pull a King and Queen(suits don't matter, nor order), with a pull each from TWO standard decks, then it would be 4/169(8/52*8/52). I'm not multiplying by two this time because they are from pulls from separate decks each time.
no, this is wrong.
imagine that the decks are green and blue.
in this case, you must actually add probabilities for both events, because they are qualitatively different events: i.e., drawing a king from the green deck and a queen from the blue deck is not the same thing as drawing a queen from the green deck and a king from the blue one.
so this would be (4/52)(4/52) + (4/52)(4/52), i.e., (green king)(blue queen) + (blue king)(green queen).

Okay, I got it now.....I forgot that the second situation was dependent on what I pulled first.
So it would be 4/169(4/52*4/52*2), and we multiply by 2 because these are different events.

lunarpower wrote:
If I want to pull a King and Queen(suits don't matter, nor order), with a pull from TWO standard decks, but this time we combine the decks and pull the cards, then it would be 60/1339(16/104*15/103*2). I would be multiplying by 2 this time since it's from the same pool.
two errors.
first, you appear to be operating under the assumption that each deck contains 8 kings and 8 queens. (weird, because you had the right numbers of kings and queens in the first part.)
second, i don't know why you decreased the numerator; that is what you'd do if you were picking two of the same card.
the numerators should be 8 and 8, not 16 and 15. otherwise, correct.
The calculation I came up with was just a whole mess of errors, not even gonna go into what the heck I was thinking lol.

So since the decks are combined, and we have 104 cards we are now trying to pull from a king and a queen as hole cards from this one giant deck, it'd be 16/1339(8/104*8/103*2), and we're still multiplying by 2 because we're picking from the same pool.
lunarpower wrote: what you don't switch is the order of the pools themselves -- in other words, once you've settled on selecting from the green deck first and from the blue deck second, you calculate ALL probabilities using that same selection order.
so, for instance, the probability of getting a king from both decks would just be (4/52)(4/52).
This blurb was extremely helpful; this is what got me over the hump!
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by lunarpower » Thu Jun 16, 2011 9:18 pm
chendawg wrote:This blurb was extremely helpful; this is what got me over the hump!
good, glad it helped.

when you have to recall this stuff in the future, try not to become overly dependent on abstract rules -- instead, try to reason by analogy.
what i mean is this:
when you look at another problem --
if you think in terms of abstractions such as "same pool", "different pools", etc., then it seems awfully hard. (i have to use these things in order to explain the principle in the first place -- this is why i use them; i'm not trying to over-complicate the issue.)
on the other hand, if you can relate the current problem to previous problems, then it's probably a bit easier.
in other words, "is this more like picking 2 cards from the same deck, or more like picking 1 card each from two different decks?" is probably easier than thinking about "pools".

try searching the forum for a problem about "joshua and jose" and trying out these methods/analogies. see how you do.
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