Author 
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jackchang1029

Post subject: Video that deals setting up 2 avg. formulas... Posted: Sat Feb 11, 2012 10:16 pm 


Students 

Posts: 6

I'm having trouble with setting up problems that deal with two average formulas (An example would be the problem listed on Pg. 104 of Guide #4 [Word Translations]).
Problem: Sam earned a $2000 commission on a big sale, raising his avg. commission by $100. If Sam's new avg. commission is $900, how many sales has he made?
Even though the solution is listed in the guide, I can't quite grasp the concept correctly and consistently applying the steps on the InAction problems (Namely, #8 & 10). Is there a video that teaches these types of questions specifically either by Ron or other instructors?
I tried searching for "Thursdays with Ron" videos by looking for ones that discussed about averages. But, they all deal with the topic of weighted averages, and I believe these problems are not considered as weighted averages since they're listed on a different section within the chapter.
If anyone can point me to the right video or tell me the name of these types of questions, it would be helpful.
Thank you!





tim

Post subject: Re: Video that deals setting up 2 avg. formulas... Posted: Sun Feb 26, 2012 4:37 am 


ManhattanGMAT Staff 

Posts: 5202 Location: Southwest Airlines, seat 21C

i don't know of any relevant videos, but the main thing to consider with problems that have two different averages is that you should often look for the total. this one is a little tricky in terms of the totals because you have to introduce variables, but the concept is the same..
_________________ Tim Sanders Manhattan GMAT Instructor
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krishnan.anju1987

Post subject: Re: Video that deals setting up 2 avg. formulas... Posted: Thu May 10, 2012 3:05 pm 


Forum Guests 

Posts: 125

I believe the solution would be something like this
Consider his average s X then, since his new average is 900 after increasing by 100, we an write the equation
X+100=900
Also, he recieved y commissions before getting the 2000 commission
then, (Xy+2000)/y+1= 900 which when translated means
800y+2000=900y+900 1100/100=y y=11
Would be great if you could confirm this.





jnelson0612

Post subject: Re: Video that deals setting up 2 avg. formulas... Posted: Sun May 20, 2012 10:21 pm 


ManhattanGMAT Staff 

Posts: 2674

krishnan, correct! Let's break it down a bit more and define some variables.
I figured that if the new average is $900, $100 more than the old average (as the problem states), then the old average is $800.
Average = Total/Number of things. Thus the old average of $800= total old revenue/number of sales. Let's call old number of sales "n". Thus, $800 = total old revenue/n. Multiply by n on both sides, and total old revenue is $800n.
New average revenue is $900. Thus, $900 = new total revenue/new number of sales. New total revenue is old revenue (800n) + new sale (2000). New number of sales is n (old number of sales) + 1 (new sale). Let's put this together:
900= (800n + 2000)/(n+1). Multiply n+1 across and you get: 900(n+1)=800n + 2000 thus 900n + 900 = 800n + 2000 100n=1100 n=11 Thus, our old number of sales is 11. Given that he just made the one new $2000 sale, total number of sales is 12.
I hope that this helps! :)
_________________ Jamie Nelson ManhattanGMAT Instructor





