Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » Official Guide Math


 Page 1 of 1 [ 4 posts ] 



  Print view Print view Previous topic Previous topic | Next topic Next topic
 

OG - PS - #197

Author Message
mww7786
 Post subject: OG - PS - #197
 Post Posted: Tue May 08, 2007 9:42 am 
I have solved this the Vendiagram way. I thought that MGMAT would have another approach. Is this so?
I hope things are going your way. Thanks for your help. :cool: Will you please throw some light on this one.

If 75% of a class answered the first question on certain test correctly, 55% answered the second question on the test correctly, and 20%
answered neither of the questions correctly, what percent answered both correctly?



a 10%
b 20%
c 30%
d 50%
e 65%
Top 
GMAT 5/18
 Post subject:
 Post Posted: Tue May 08, 2007 11:45 pm 
mww7786,

Overlapping sets would be the quickest way to solve this problem. It took me approx. 45s.

I used a table with the column headings = 1st Correct, 1st Not Correct, Total and the row headings 2nd Correct, 2nd Not Correct, Total. This gave me a table with 9 "boxes" to fill. Simply fill in the boxes with the given data:

Total/Total = 100, 1st Correct/Total = 75, Total/2nd Correct = 45 and 1st Not Correct/2nd Not Correct = 20.

From here you can fill in every remaining combination, using addition and subtraction.

Hope this helps!

By the way, the answer is D, 50%.
Top 
dbernst
 Post subject:
 Post Posted: Wed May 09, 2007 11:02 pm 
Offline
ManhattanGMAT Staff


Posts: 304
Overlapping sets is a great approach. I also have an even quicker formula should you really feel the need for speed. In one group that is broken down into two overlapping sets, the following formula applies.

Total = Group 1 + Group 2 + Neither - Both

In this problem, let's make the total 100. Thus,

100 = 75 + 55 + 20 - B.

B = 50!
Top 
GMAT 5/18
 Post subject:
 Post Posted: Wed May 09, 2007 11:25 pm 
Wow, I stand corrected!

Overlapping sets certainly is not the quickest way to solve this problem. Thanks Dan, for enlightening us!

Total = Group 1 + Group 2 + Neither - Both.

Got it! ;)
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 4 posts ] 





Who is online

Users browsing this forum: No registered users and 0 guests

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to: