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Average height

by snapplesf » Mon Mar 12, 2007 9:12 am
Hello,

This is my first post. So please correct me if I'm not following an expected procedure -

DS Question - Source: - Practise Test from mba.com
------------------------------------------------------------

What is the average height of the n people in a certain group ?

1) The avg. height of the n/3 tallest people in the group is 6ft and the avg. height of rest of the people is 5ft.

2)The sum of heights of the n people is 178.





Answer is A.

I picked C. I don't understand how you can arrive at a figure using just A. Using A&B , I could set up an equation to solve for n.
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Re: Average height

by gabriel » Mon Mar 12, 2007 1:21 pm
snapplesf wrote:Hello,

This is my first post. So please correct me if I'm not following an expected procedure -

DS Question - Source: - Practise Test from mba.com
------------------------------------------------------------

What is the average height of the n people in a certain group ?

1) The avg. height of the n/3 tallest people in the group is 6ft and the avg. height of rest of the people is 5ft.

2)The sum of heights of the n people is 178.





Answer is A.

I picked C. I don't understand how you can arrive at a figure using just A. Using A&B , I could set up an equation to solve for n.

The q shuld be posted in the DS section....

nywayz.... the q is based on weighted average .... read this for more information on weighted average https://en.wikipedia.org/wiki/Weighted_mean ..... since the ratio of the people who are 6 ft and the pople who are 5 ft is known we can find the average ht of the entire group.... so the answer in this case wuld be 6*1/3+5*2/3.... so the answer is a...
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thanks !

by snapplesf » Mon Mar 12, 2007 2:20 pm
Thanks gabriel !
Had no idea that weighted averages were tested on the gmat.
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by shobhits » Sat Dec 25, 2010 9:40 pm
The formula to be used is Average height = Sum of heights of all people / number of people.

Statement 1:

Avg. height of the n/3 tallest people = S1 / (n/3) = 6, where S1 is the sum of the heights of the n/3 tallest people.
That gives us S1 = 2n.

Avg. height of the rest of the 2n/3 people = S2 / (2n/3) = 5, where S2 is the sum of the heights of the remaining 2n/3 people.
That gives us S2 = 10n/3.

Average is S1 + S2 / n = [2n + (10n/3)] / n. 'n' will cancel out and you can find the average.


Statement 2:
INSUFFICIENT. Doesn't give the value of n.

And yes, weighted averages are tested on the GMAT.
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by HPengineer » Sat Dec 25, 2010 9:59 pm
I noticed weighted average but made a poor algebra mistake..

((n/3)x6 + (n- n/3)5)/n = Avg


I failed to notice that the Ns will cancel out... moved to quickly and assumed two unknowns = insuffcient
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