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A radio station has to choose three days of the seven

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Magoosh GMAT Instructor
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A radio station has to choose three days of the seven [#permalink] New post 03 Dec 2012, 11:02
Expert's post
00:00
A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

94% (01:56) correct 6% (00:50) wrong based on 32 sessions
A radio station has to choose three days of the seven in a week to broadcast a certain program, and that set will repeat each week. The program can be broadcast equally on any of the seven weekdays —- weekdays vs. weekends don’t matter at all —- nor does it matter whether the days the program airs are adjacent or not. Absolutely any three of the seven weekdays can be chosen. How many different three-day combinations of the seven weekdays can be constructed?
(A) 9
(B) 15
(C) 21
(D) 35
(E) 56


For tips on calculating the answer to questions like this without a calculator, see this post:
http://magoosh.com/gmat/2012/gmat-math- ... binations/
Experts, feel free to share you own non-calculator tips for calculating nCr values.

Mike :-)
[Reveal] Spoiler: OA

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Re: A radio station has to choose three days of the seven [#permalink] New post 03 Dec 2012, 23:59
mikemcgarry wrote:
A radio station has to choose three days of the seven in a week to broadcast a certain program, and that set will repeat each week. The program can be broadcast equally on any of the seven weekdays —- weekdays vs. weekends don’t matter at all —- nor does it matter whether the days the program airs are adjacent or not. Absolutely any three of the seven weekdays can be chosen. How many different three-day combinations of the seven weekdays can be constructed?
(A) 9
(B) 15
(C) 21
(D) 35
(E) 56


For tips on calculating the answer to questions like this without a calculator, see this post:
http://magoosh.com/gmat/2012/gmat-math- ... binations/
Experts, feel free to share you own non-calculator tips for calculating nCr values.

Mike :-)


Pick 3 days out of 7.. 7C3 = 7!/(4!*3!) = 35

Ans D it is.
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Re: A radio station has to choose three days of the seven [#permalink] New post 27 Dec 2012, 01:38
I used the ANAGRAM technique to answer this question:

D1 D2 D3 D4 D5 D6 D7
N N N Y Y Y N


ANAGRAM gets the permutation of the options and then divide it by the number of selected and the number of not selected. In our case, we are looking for the permutation of 7 days divided by 3 selections and 4 non-selections.

=\frac{7!}{4!3!}=7*5=35

Answer: D
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Re: A radio station has to choose three days of the seven   [#permalink] 27 Dec 2012, 01:38
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