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permutation and combination

Expert replies
by anjaligeorge1 » Sun Jun 29, 2008 4:52 am
Six mobsters have arrived at the theater for the premiere of the film “Goodbuddies.” One of the mobsters, Frankie, is an informer, and he's afraid that another member of his crew, Joey, is on to him. Frankie, wanting to keep Joey in his sights, insists upon standing behind Joey in line at the concession stand, though not necessarily right behind him. How many ways can the six arrange themselves in line such that Frankie’s requirement is satisfied?


6
24
120
360
720
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Source: — Problem Solving |

by debarshi7 » Sun Jun 29, 2008 5:23 am
is te answer 360
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by anjaligeorge1 » Sun Jun 29, 2008 5:35 am
the answer is 360 can u explain how u solved it .
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by Ian Stewart » Sun Jun 29, 2008 5:43 am
If you line them up randomly, there's a 50% chance Frankie will be ahead of Joey in line, and a 50% chance Frankie will be behind Joey in line. With no restrictions, we can line the people up in 6*5*4*3*2*1 = 720 ways, and in exactly half of these lineups, Frankie is behind Joey. 720/2 = 360.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by younggun044 » Sun Jun 29, 2008 6:35 am
any other method used by anyone?
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by younggun044 » Sun Jun 29, 2008 6:41 am
any other method used by anyone?
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by atlantic » Mon Jun 30, 2008 5:38 am
younggun044,

You have to fix Joey in all the possible positions and calculate the possible arrangements for the remaining people.

1st: Joey is on the first position

Therefore no matter what is the position of Frankie he'll be behind Joey. Since Joey is fixed, the remaining five have 5! ways of being arranged.

2nd: Joey is in the second position

Frankie must be on position nr. 3, 4, 5 or 6, meaning 4 possible positions. The remaining 4 people must be arranged in the remaining 4 positions. So 4*4!

3rd: Joey is in third position

Frankie must be on position nr. 4, 5 or 6, meaning 3 possible positions. The remaining 4 people must be arranges in the remaining 4 positions. So, 3*4!

4th: Joey is in fourth position

Frankie must be on position nr. 5 or 6, 2 possible positions. The remaining 4 people can be arranged in the 4 remaining positions. So 2*4!

5th: Joey is in fifth position

Frankie must be on position nr. 6, one possible position. The remaining 4 people can be arranged in the remaining 4 positions, so 4!

Joey cannot be on position nr. 6 because this is the last position, meaning that Frankie could not be behind him.

So, 5! + 4*4! + 3*4! + 2*4! + 4! = 360

Hope it helps.
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by younggun044 » Tue Jul 01, 2008 2:34 am
yup...ofcourse it does!!
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