Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » General Math Questions


 Page 1 of 1 [ 3 posts ] 



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

Evenly Spaced Set Average Question

Author Message
ellabaro
 Post subject: Evenly Spaced Set Average Question
 Post Posted: Wed Jul 06, 2011 11:31 pm 
Offline
Students


Posts: 3
Hi, I took a Manhattan CAT exame and came across the following question:
Given that a is the average (arithmetic mean) of the first nine positive multiples of six and b is the median of the first twelve positive multiples of six, what is the ratio of a to b?
Correct Answer: 10:13

I understand how to complete this problem the long way; however, when I tried taking a shortcut that I learned I did not come up with the correct answers. To find the average of the first nine positive multiples of six I subtracted the ninth multiple of 6 (54) from the first multiple of 6 (6) = 48 and divided 48 by 2 - 24. When I took the convential way of finding the average, my answer is 30 and leads to me to the correct answer choice.

What am I doing incorrectly in the shortcut version? I can't figure it out...
Top 
mercytron
 Post subject: Re: Evenly Spaced Set Average Question
 Post Posted: Thu Jul 07, 2011 12:13 pm 
Offline
Course Students


Posts: 2
The average of evenly spaced sets is (first number + last number)/2.

In this case for first 9 multiples og 6, average will be (6+54)/2 = 30.
Top 
jnelson0612
 Post subject: Re: Evenly Spaced Set Average Question
 Post Posted: Thu Jul 14, 2011 11:56 am 
Offline
ManhattanGMAT Staff


Posts: 1857
Thanks mercytron!

_________________
Jamie Nelson
ManhattanGMAT Instructor
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 3 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: