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How many different ways can 2 students be seated in a row of

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VP
Joined: 21 Jul 2006
Posts: 1491
How many different ways can 2 students be seated in a row of [#permalink]

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09 Nov 2008, 11:00
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How many different ways can 2 students be seated in a row of 4 desks, so that there is always at least one empty desk between the students?

a) 3
b) 2
c) 4
d) 6
e) 12

Well, the OA to this is D. It's easy because the number is small and thus you can manually do the math. However, what I want to know is the appropriate formula to use in case I come across a similar question on the real GMAT, but with a much bigger number that may be time consuming to do the math manually.

Thanks!
SVP
Joined: 17 Jun 2008
Posts: 1504
Re: PS: Combination. Need the appropriate formula [#permalink]

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09 Nov 2008, 11:11
I am not sure about the formula. But, in such questions, visual representation helps me understand better.
VP
Joined: 21 Jul 2006
Posts: 1491
Re: PS: Combination. Need the appropriate formula [#permalink]

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09 Nov 2008, 11:19
well, there must be a formula because we can't rely on the visual presentation should we have a huge and complicated number. What would you do in that case? so there must be a formula. If anyone knows how to answer this by using a formula, then it will be very useful to know!
thanks
Re: PS: Combination. Need the appropriate formula   [#permalink] 09 Nov 2008, 11:19
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