Posted by: AC | May 15, 2009

GMAT Hacks

Recently while studying for the GMATs, I’ve been running into a problem specifically when it comes to quantitative questions.  It is an fairly common obstacle that lots of people face:   I arrive at the correct answers, but I know I take way too long to get there.

It was then that a friend referred me to GMAT,  a website by the author of the Total GMAT books. Jeff Sackman.

There are several articles illustrating his tips and tricks on the GMATs that I have found helpful.  Most recently I read  a great article on doing weighted averages , problems that I was getting right but I felt like they were taking too long.

Here is a sample problem and Jeff’s explanation:

If Jason purchased two suits for $179 each and three suits for $189 each, what is the average price Jason paid for each suit?

(A) $183.00
(B) $184.00
(C) $185.00
(D) $185.50
(E) $186.50

Jeff explains that most of us would probably set up the problem like this:

(179)(2) + (189)(3)

…and eventually arrive at the right answer:   (C), $185.00.

Jeff’s Approach

He writes ” Now think of a different number line: this time between 0 and 10, inclusive. If you were calculating the weighted average of 2 0’s and 3 10’s, the average would nudge toward 10. The calculations are much simpler in that case:

(0)(2) + (10)(3)

30 divided by 5 is 6. Again, despite the fact that we’re looking at a number line 179 points lower than the original, the weighted average is exactly 6 larger than the lower number and 4 smaller than the higher number. This is no accident. In a matter of speaking, weighted averages don’t care what the end points are, they just care about the weights.

By now, it may be clear how to apply this to do the example problem above more quickly. Instead of using 179 and 189 as your datapoints, use 0 and 10. Find the weighted average of 2 0’s and 3 10’s, then add 179. ”

Cue: light bulb on!  Thank you, Jeff!


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