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 Post subject: Test:Q-15
 Post Posted: Wed Oct 15, 2008 2:59 pm 
Code:
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?
A.    6
B.   24
C. 120
D. 360
E.  720


I got this question wrong. Need help to solve the problem. Guys, can you pls help me by giving an approach to attack this type of problem?

Wish to hear from you guys ...


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 Post subject:
 Post Posted: Tue Oct 21, 2008 3:04 pm 
I approach questions like these as follows (counting method)

Draw (!!!) the situation

X1------X2------X3------X4------X5------X6

where X is the position of the mobsters in the queue. Say the queue starts from left.

Condition is that Frankie (F) behind Joey (J) but not necessarily the next one behind him

Case 1: Joey is the first person in the line. thus frankie can take any position from X2 to X6.

J------X2------X3------X4------X5------X6

Joey can occupy X1 position in 1 way, Frankie can occupy 5 positions (X2 to X6) and the remaining 4 mobsters can occupy remaining 4 places in 4! ways.
So total number of ways = 1 x 5 x 4!

Case 2: Joey is the second person in the line. thus frankie can take any position from X2 to X6.

X1------J------X3------X4------X5------X6

Joey can occupy X2 position in 1 way, Frankie can occupy 4 positions (X3 to X6) and the remaining 4 mobsters can occupy remaining 4 places in 4! ways.
So total number of ways = 1 x 4 x 4!

Case 3: Joey is the first person in the line. thus frankie can take any position from X2 to X6.

X1------X2------J------X4------X5------X6

Joey can occupy X3 position in 1 way, Frankie can occupy 3 positions (X4 to X6) and the remaining 4 mobsters can occupy remaining 4 places in 4! ways.
So total number of ways = 1 x 3 x 4!

Case 4: X1------X2------X3------J------X5------X6

Joey can occupy X4 position in 1 way, Frankie can occupy 2 positions (X5 or X6) and the remaining 4 mobsters can occupy remaining 4 places in 4! ways.
So total number of ways = 1 x 2 x 4!

Case 5: X1------X2------X3------X4------J------X6

Joey can occupy X5 position in 1 way, Frankie can occupy 1 position (X6) and the remaining 4 mobsters can occupy remaining 4 places in 4! ways.
So total number of ways = 1 x 1 x 4!


Total 5 cases hence add all the cases

= 1 x 5 x 4! + 1 x 4 x 4! + 1 x 3 x 4! + 1 x 2 x 4! + 1 x 1 x 4!
= 1 x ( 5 +4 +3 +2 +1 ) x 4!
= 1 x 15 x 4! (=24)
= 360

Is 360 the answer ?


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 Post subject: Corrections
 Post Posted: Tue Oct 21, 2008 3:07 pm 
Case 2: Joey is the second person in the line. thus frankie can occupy 4 positions from X3 to X6.

X1------J------X3------X4------X5------X6

Joey can occupy X2 position in 1 way, Frankie can occupy 4 positions (X3 to X6) and the remaining 4 mobsters can occupy remaining 4 places in 4! ways.
So total number of ways = 1 x 4 x 4!

Case 3: Joey is the third person in the line. thus frankie can occupy 3 positions from X4 to X6.


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 Post subject:
 Post Posted: Sat Nov 15, 2008 7:45 am 
Offline
ManhattanGMAT Staff


Posts: 7146
this problem has a lot of mileage on it already in this forum.

just type "mobsters" into the search box, and you'll find the following:
http://www.manhattangmat.com/forums/six ... t1357.html
http://www.manhattangmat.com/forums/post2952.html

check those out. if you still have questions, then post them - preferably in one of those threads, as it's nice to have everything in one place.


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