how many of the students in a certain class

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ceo1000: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Wed Jan 26, 2011 7:40 am

how many of the students in a certain class

by ceo1000 » Thu May 12, 2011 6:38 am

how many of the students in a certain class are taking both history and science class?
(1) Of all the students in the class, 50 are taking a history course
(2) Of all the students in the class, 70 are taking a science course

I thought C but it was E. Could somebody explain to me why?
Is it because the total number of students wasn't given?

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Source: Beat The GMAT — Data Sufficiency | Thu May 12, 2011 6:38 am

vineeshp: Legendary Member; Posts: 965; Joined: Thu Jan 28, 2010 12:52 am; Thanked: 156 times; Followed by:34 members; GMAT Score:720

by vineeshp » Thu May 12, 2011 7:23 am

Well.

The class can have 4 sets of Students.

1. Take only history:
2. Take only Science:
3. Take both.
4. Take neither of the two.

1, WE have been given 50 but we havent been told how many of them also do Science.
2. WE have been given 70 but we havent been told how many of them also do history.

Try drawing 2 intersecting circles and representing the above data from 1-4.

The Set theory in OG should help you with this.

Vineesh,
Just telling you what I know and think. I am not the expert.

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GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Thu May 12, 2011 9:16 am

ceo1000 wrote:how many of the students in a certain class are taking both history and science class?
(1) Of all the students in the class, 50 are taking a history course
(2) Of all the students in the class, 70 are taking a science course

I thought C but it was E. Could somebody explain to me why?
Is it because the total number of students wasn't given?

You can also solve this Overlapping Sets question by using the Double Matrix method.
I wrote an article about this stratgey last week for BTG: https://www.beatthegmat.com/mba/2011/05/ ... question-1

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com