Telling permutation problems from combinations problems

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In the document downloadable from here:

https://www.beatthegmat.com/difficult-gm ... s-t78.html

this question appears:

20. In how many ways can one choose 6 cards from a normal deck of cards so as to have all suits present?

a. (13^4) x 48 x 47
b. (13^4) x 27 x 47
c. 48C6
d. 13^4
e. (13^4) x 48C6


I know this isn't an official GMAT question, but would you read that as implying that order matters, or that order does not matter?

I read it as:

How many ways can you draw 6 cards from 52 and have at least one card of each suit?

But it could also be read as:

How many 6-card hands from a standard deck contain at lease one card of each suit? The answer given in the text was [spoiler]13^4 *48*47[/spoiler].


I read it the first way, and tried to solve it like this:

There are two possible types of suit arrangement:

- 3 of one, and 1 of each of the three others
- 2 of one, 2 of another, and one each of the other two.

The first arrangement can occur in (4C1) four ways, the second arrangement can occur in (4C2) six ways.

So total possibilities =

4 * (13C3 * 13^3) + 6 * (13C2 * 13C2 * 13^2)

= 4*(13 * 2 * 11 * 13^3) + 6 (13 * 6 * 13 * 6 * 13^2)

= 88*(13^4) + 216(13^4)

= 304 * (13^4)

Does that look right, given my assumption about the meaning of the question?
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by betamax » Sat Feb 07, 2009 11:51 pm
I go with a. 13c1*13c1*13c1*13c1 * 48c1 *47c1

Once you have the 4 suits, the remaining 2 cards don't matter. You just need at least 1 of each suit. Each other card thereafter then just increases the number of any of the 4 suits you already have. So you choose the next card of the remaining 48 and the last of the remaining 47. Or you can just use 48p2.

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by lunarpower » Sun Feb 08, 2009 11:54 am
i'm with cjb on this one. well solved.

13 x 13 x 13 x 13 x 48 x 47 is definitely the wrong answer to this problem, because it assumes that "order matters".
when you multiply consecutive numbers of possibilities, you are automatically implying that "order matters" in the problem.
if this is the "official answer" in the source, then the source is simply wrong.

sometimes, if "order" actually doesn't "matter" in the problem, you can fix the discrepancy simply by dividing by the appropriate factorial(s). however, that's not the case in this problem, because there are different possibilities (as spelled out by cjb), with correspondingly different numbers of interchangeable elements. specifically, the cases in which 3 cards have the same suit correspond to different "factorial corrections" than do the cases in which there are 2 cards each of 2 suits, so it's impossible to divide the entire product 13 x 13 x 13 x 13 x 48 x 47 by any single factorial to adjust for "order" not "mattering".

this means that the only way to solve this problem correctly is to take the sort of approach followed by cjb above. this seems to be an unacceptably high amount of work for a gmat problem, but it's still good to study the individual pieces of cjb's approach.
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by lunarpower » Sun Feb 08, 2009 11:55 am
by the way, you may want to consider posting a link to this problem in the Problem Solving sub-section.

this "general math" area is meant for math issues that are general or wider-scale, not for particular problems. if you post problems here in no-man's-land, they won't get a lot of attention.
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by Ian Stewart » Sun Feb 08, 2009 1:15 pm
Yeah, I interpreted the problem the same way as you did, James, and the answer given is not correct, as Ron points out. Your approach looks perfect on a quick read, and I don't think there's a more efficient path to the answer here. There are at least a couple of suspect problems in the document you're referring to, so I wouldn't trust every answer given. While this question is a bit too crazy to be a realistic GMAT problem regardless, it is also ambiguous about whether different selection orders should be counted as though they were distinct selections, and the real GMAT is never ambiguous about whether order is important.
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