l0rrie wrote:

Hi,

thanks for your reply.. It's the second part I just can't seem to get because if you choose 3 out of 12 and you only have 3 spots that can be filled.. Why is it a combination because if either of those 12 take a seat in the delegation then only 11 can still be seated next to the other one eventually leading to 12!/9!

If the 3 are duplicates, why, for example if you have 3 chairs and 7 people, that is not a duplicate.. Either way it also wouldnt matter who sits on what chair out of the 3 people that eventually will take a seat.. But, because if one person takes a seat on the first chair then there are only 2 chair left for 6 people etc. etc.

I think my main problem is that I just can't seem to get the difference behind the logic of these kind of questions.. Most of the time practicing, I end up thinking its a permutation while its a combination

I agree with gmat1220.

Permutations:

Arrangement and Order matters.

Let me give you a small example.

How many words can you form with two letters A and B, if the letters can't be repeated.

AB

BA

2 i.e. 2P2 = 2!(Permutation)

How many teams of 2 can you select with two girls A and B.

Can it be

AB

BA

No... AB and BA are both same teams. Order doesn't matter. If AB are used, that should be it.

The answer is 1.

2C2 = 1(Combination)

I don't get these concepts many times myself. Tell you my trick; I have by heart most of the patterns used in the combinations and permutations. Consequence: when I encounter a really witty question, I just scratch my head and roll my eyes. Then again, not many of the questions are witty.

Say for the above question, I would blindly solve it using combination. I won't even bother to look for alternative approaches. I know there are x people, y needs to be selected: ah.. selected(Combination). I got you Mr question.

3 out of 12 AND 2 out of 11

12C3 * 11C2 (I read it somewhere: AND is multiplication and OR is addition)

If you start with the pattern recognition in these questions like a robot, you will do just fine initially. The interesting part is: after solving many questions, you will automatically start getting the underlying logic behind the solution. Don't ask me how; just try it yourself. But, if you are a guy who just abhors to solve a question without proper visualization; please ignore my suggestion.

_________________

~fluke

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