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lawyers

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Newbie | Next Rank: 10 Posts
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Joined: Mon Mar 23, 2009 4:50 am

lawyers

by gmattaker33 » Mon Mar 23, 2009 4:54 am
28 lawyers from 3 firms X, Y, Z. They can either belong to only 1 firm or belong to the three firms at the same time. X firm has 10 lawyers, Y has 11 lawyers and Z has 13 lawyers. How many people only belong to Z?

Solution: 10

10+11+13-2n=28 ----〉why 2n and not only n??

N=3

13-n=10
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Junior | Next Rank: 30 Posts
Posts: 15
Joined: Fri Mar 20, 2009 8:59 am
GMAT Score:600

Re: lawyers

by 2ndShot » Mon Mar 23, 2009 5:10 am
gmattaker33 wrote: 10+11+13-2n=28 ----〉why 2n and not only n??
Because

n=number of lawyers belonging to all three firms at the same time
10-n=lawyers only in X
11-n=lawyers only in Y
13-n=lawyers only in Z

Therefore the total number of lawyers is

(10-n)+(11-n)+(13-n)+n = 10+11+13-3n+n = 10+11+13-2n =28
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