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Joined: 05 Nov 2005
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Greg is renting movies from the video store. He must choose
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27 Nov 2005, 19:38
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Greg is renting movies from the video store. He must choose 3 videos from a list of 10 videos and decide in which order to watch them. How many schedules of videos can he create?
A. 30
B. 70
C. 700
D. 720
E. 1000 == Message from the GMAT Club Team == THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION. This discussion does not meet community quality standards. It has been retired. If you would like to discuss this question please repost it in the respective forum. Thank you! To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative  Verbal Please note  we may remove posts that do not follow our posting guidelines. Thank you.



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I believe its a common trap to trick the testtaker to do a twostep solution (Well, I was tricked once before ). Assuming the videos are numbered 1,2,3,4,5,6,7,8,9,10. It doesn't matter if we pick 1,2,3 and then order them to get 1,2,3; 3,2,1; 2,3,1; 3,1,2; 2,1,3;1,3,2. It's the same as ordering 1,2,3 from the 10 videos.
So, to solve, just permute 3 videos from the selection of 10.
10P3 = 10!/3!7! = 720



Intern
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But my understanding was....
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28 Nov 2005, 03:29
ywilfred wrote: I believe its a common trap to trick the testtaker to do a twostep solution (Well, I was tricked once before ). Assuming the videos are numbered 1,2,3,4,5,6,7,8,9,10. It doesn't matter if we pick 1,2,3 and then order them to get 1,2,3; 3,2,1; 2,3,1; 3,1,2; 2,1,3;1,3,2. It's the same as ordering 1,2,3 from the 10 videos. So, to solve, just permute 3 videos from the selection of 10. 10P3 = 10!/3!7! = 720
My understanding was if the order doesnt matter than you are supposed to use combination. So according to your example, we should have used combination rather than permutation? I think according to the question, it does matter how you watch the first three and thats why you used permutation.



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Can you explain why you used combination?
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28 Nov 2005, 03:31
gamjatang wrote: 10C3 = 120
3! = 6
120 * 6 = 720
Can you explain how you got the answer?
Thanks



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Re: Can you explain why you used combination?
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28 Nov 2005, 03:39
faisalt wrote: gamjatang wrote: 10C3 = 120
3! = 6
120 * 6 = 720 Can you explain how you got the answer? Thanks
Cases of picking 3 movies out of 10 movies
= 10C3
Cases of arranging 3 movies
= 3!
Cases of picking 3 movies and arranging 3 movies
= 10C3 * 3!
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Director
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I agree with Gamjatang.
Wilfred, Can you elaborate more on your explanation? Thanks



Intern
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I kinda disagree with Gamjatang...
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28 Nov 2005, 14:30
anandsebastin wrote: I agree with Gamjatang. Wilfred, Can you elaborate more on your explanation? Thanks
Greg is renting movies from the video store. He must choose 3 videos from a list of 10 videos and decide in which order to watch them. How many schedules of videos can he create?
I think we are supposed to use permutation, as the order in which movies are being watched matters. It is not same as watching {1, 2, 3} as {3,2,1}. I agree with the Wilfred but his explanation is kinda conflicting.
Thanks,



Senior Manager
Joined: 27 Jun 2005
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Location: MS

since order matters so its permutations ..
10P3 = 720



Intern
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Re: PSProbability
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07 May 2009, 10:49
The trick relies on the assumption that order does NOT matter. I mean, we think we have to choose any 3 movies out of 10 and after that arrange them in schedules. From the question "How many different schedules of videos can he create?" we should notice that order DOES matter so we shortcut by picking the movies in the different orders (Permutation) we can schedule them since the beginning.



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Joined: 28 Jan 2004
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Location: India

Re: PSProbability
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13 May 2009, 21:22
I am a bit confused here....
see we are saying that out of 10 we need to select 3.
So we can have 720 sets (or ways) containing 3 movies each {S1},{S2},{S3}............{S720}. Now when he play set S1 he have 6 choices as there are 3 movies in this set.
So the answer should be 720*6 = 4320



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Re: PSProbability
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13 May 2009, 22:23
mdfrahim wrote: I am a bit confused here....
see we are saying that out of 10 we need to select 3.
So we can have 720 sets (or ways) containing 3 movies each {S1},{S2},{S3}............{S720}. Now when he play set S1 he have 6 choices as there are 3 movies in this set.
So the answer should be 720*6 = 4320 You can only get 720 sets if you consider the order in each set. If you ignore the order, then you only get 120 sets.



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Re: PSProbability
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15 May 2009, 12:35
faisalt wrote: Greg is renting movies from the video store. He must choose 3 videos from a list of 10 videos and decide in which order to watch them. How many schedules of videos can he create?
A. 30 B. 70 C. 700 D. 720 E. 1000 1. 10x9x8 = 720 2. 10c3 x 3p1 = (3!) (10x9x8x7!)/[(103)!3!] = 720
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Re: Greg is renting movies from the video store. He must choose
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28 Jan 2018, 22:06
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Re: Greg is renting movies from the video store. He must choose &nbs
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