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## OG13 - Quant Review - DS - 49

tagged by: Brent@GMATPrepNow

This topic has 3 expert replies and 3 member replies
vittovangind Junior | Next Rank: 30 Posts
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#### OG13 - Quant Review - DS - 49

Sun Jan 19, 2014 2:06 pm
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
Hello,

"49. How many directors of both Company K and Company R?

(1) Thee were 17 directors present at joint meeting of the directors of Company K and Company R, and no directors were absent.

(2) Company K has 12 directors and Company R has 8 directors."

Please could someone rephrase the solution taking (1) and (2) together. Specificall, I don't understand why the 3 peoples missing at the joint meeting must be joint directors.

Many Thanks,

Vitto[/list]

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Rich.C@EMPOWERgmat.com Elite Legendary Member
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Sun Jan 19, 2014 4:24 pm
Hi Vitto,

This DS question is based on a variation of the "overlapping sets" concept that you'll likely see on the GMAT once or twice. There are several different ways to solve this problem, but since this is a simple variation, I'll show you the simple math.

The idea is this: If a person is a member of BOTH groups, then that person has been "counted" twice (once for the first group and once for the second). When calculating the total number of people, you're NOT supposed to count people twice, so you have to mathematically remove that "second count."

Here's a simple example:
3 people total
1 person in group A
1 person in group B
1 person in BOTH group A and B

Total in group A = 2
Total in group B = 2
But that DOES NOT mean that there are 4 people; there are only 3.

In this type of question, the "math" formula is Total = Group A + Group B - BOTH = 2 + 2 - 1 = 3 people total

The same logic applies in this question.
Total = 17
Company K = 12
Company R = 8
Both = ?

Total = CompK + CompR - BOTH
17 = 12 + 8 - BOTH
17 = 20 - BOTH
3 = BOTH

GMAT assassins aren't born, they're made,
Rich

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Patrick_GMATFix GMAT Instructor
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Tue Jan 21, 2014 1:46 pm
The solution below is taken from the GMATFix App.

-Patrick

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### GMAT/MBA Expert

Brent@GMATPrepNow GMAT Instructor
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Tue Jan 21, 2014 4:17 pm
vittovangind wrote:
Hello,

How many directors of both Company K and Company R?

(1) Thee were 17 directors present at joint meeting of the directors of Company K and Company R, and no directors were absent.

(2) Company K has 12 directors and Company R has 8 directors."

Another approach is to use the Double Matrix Method. This technique can be used for most questions featuring a population in which each member has two characteristics associated with it.
Here, we have a population of directors, and the two characteristics are:
- director at company K or not a director at company K
- director at company R or not a director at company R

Once you're familiar with the Double Matrix Method, you can attempt the original question above, as well as these additional practice questions:

Easy Problem Solving questions
- http://www.beatthegmat.com/finance-majors-non-finance-majors-overlapping-set-question-t167425.html

Medium Problem Solving questions
- http://www.beatthegmat.com/probability-question-t273360.html
- http://www.beatthegmat.com/posted-speed-limit-t272374.html
- http://www.beatthegmat.com/motel-t271938.html
- http://www.beatthegmat.com/of-the-applicants-passes-a-certain-test-15-t270255.html
- http://www.beatthegmat.com/opening-night-and-late-patrons-t264869.html
- http://www.beatthegmat.com/ds-french-japanese-t222297.html

Difficult Problem Solving questions
- http://www.beatthegmat.com/ratio-problem-t268339.html
- http://www.beatthegmat.com/overlapping-sets-question-t265223.html
- http://www.beatthegmat.com/fractions-t264254.html
- http://www.beatthegmat.com/overlapping-sets-t264092.html
- http://www.beatthegmat.com/mba/2011/05/09/random-double-matrix-question-2

Easy Data Sufficiency questions
- http://www.beatthegmat.com/for-what-percent-of-those-tested-for-a-certain-t270596.html
- http://www.beatthegmat.com/ds-quest-t187706.html

Medium Data Sufficiency questions
- http://www.beatthegmat.com/sets-matrix-ds-t271914.html
- http://www.beatthegmat.com/each-of-people-voted-once-in-an-election-x-t271375.html
- http://www.beatthegmat.com/a-manufacturer-t270331.html
- http://www.beatthegmat.com/in-costume-for-halloween-t269355.html
- http://www.beatthegmat.com/mba/2011/05/05/random-double-matrix-question-1

Difficult Data Sufficiency questions
- http://www.beatthegmat.com/double-set-matrix-question-t271423.html
- http://www.beatthegmat.com/sets-t269449.html
- http://www.beatthegmat.com/mba/2011/05/16/random-double-matrix-question-3

Cheers,
Brent

_________________
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Last edited by Brent@GMATPrepNow on Thu May 08, 2014 1:08 pm; edited 1 time in total

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abhigupta85 Newbie | Next Rank: 10 Posts
Joined
08 May 2014
Posted:
2 messages
Thu May 08, 2014 10:08 am
Brent

Can you please draw double matrix for this question.

Thanks
Abhishek

abhigupta85 Newbie | Next Rank: 10 Posts
Joined
08 May 2014
Posted:
2 messages
Thu May 08, 2014 10:18 pm

### GMAT/MBA Expert

Brent@GMATPrepNow GMAT Instructor
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Fri May 09, 2014 1:55 am
abhigupta85 wrote:
Perfect! The tricky part is recognizing that there are zero attendees in the bottom-right box.

Cheers,
Brent

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