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mba.com question on combinations

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shantascherla
 Post subject: mba.com question on combinations
 Post Posted: Mon Mar 21, 2011 12:54 pm 
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Course Students


Posts: 21
The lunch menu at a restaurant consists of 4 different entrees and 5 different side dishes

If a meal consists of 1 main course and 2 different side dishes how many combinations can be chosen

10
20
40
80
100

Answer 40

I approached this question in the prep using the slot machine approach

So for the maincourse he has 4 options and for the first side dish 5 and the second dish 4

So it came to 80

I then tried the combn approach for the side dishes C5/2 and reached to a 40 answer which matches the solution

My question is why the slot approach didnt work here ?
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sfbay
 Post subject: Re: mba.com question on combinations
 Post Posted: Mon Mar 21, 2011 5:38 pm 
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Forum Guests


Posts: 26
Location: San Francisco
you can watch "study hall with Ron" December 3, 2009 for a refresher on combinatorics.

http://www.manhattangmat.com/thursdays-with-ron.cfm



slot method - you need to divide the side dishes by 2! to eliminate double counting of 'peas and carrots' or 'carrots and peas'
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shantascherla
 Post subject: Re: mba.com question on combinations
 Post Posted: Mon Mar 21, 2011 11:50 pm 
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Course Students


Posts: 21
Thanks got it
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jnelson0612
 Post subject: Re: mba.com question on combinations
 Post Posted: Wed Mar 23, 2011 7:44 pm 
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ManhattanGMAT Staff


Posts: 1857
Thanks munny.

_________________
Jamie Nelson
ManhattanGMAT Instructor
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