Estimating Percents. Please help :(

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meh: Junior | Next Rank: 30 Posts; Posts: 11; Joined: Mon Jun 17, 2013 7:04 am

by meh » Mon Dec 16, 2013 6:28 am

Hi there,

I am having major difficulties estimating percents QUICKLY.

My difficulty is that, it is impossible to know which method to use without applying it first, and even once you apply it, even the 10's digit may be wrong!

Take 47/58.

Method 1: 5/6 = about 83%

Method 2: 46/58 = 23/29 = about (23/30) x (3/3) = 69/90 = about 76%

Method 3: (47/60) x (1.6/1.6) = 29+47 = abooout 78%

Any tips?

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Source: Beat The GMAT — Problem Solving | Mon Dec 16, 2013 6:28 am

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Mon Dec 16, 2013 8:04 am

Hi Meh,

It's important to note that the GMAT test-makers are not trying to identify (and reward) the human calculators out there (Here's an article I wrote on this topic: https://www.beatthegmat.com/mba/2012/02/ ... test-maker)

So, in the vast majority of cases, you shouldn't have to perform a lot of tedious calculations. Instead, the answer choices will either be sufficiently spread apart to allow for some very rough estimation, or there will be another approach that's just as easy.

For example, let's say you need to convert 13/48 into the nearest percent, and the answer choices are:
A) 24
B) 27
C) 32
D) 33
E) 36

One approach here is to recognize that 12/48 = 25%, so 13/48 must be a little bit bigger than 25%.
Choose B

Alternatively, we might try to rewrite 13/48 as a fraction with 100 in the denominator (since fractions with 100 in the denominator are VERY EASY to covert to percents).
So, we want 13/48 = ?/100
To create this equivalent fraction, we must multiply numerator and denominator by a number a little bit bigger than 2 (since 2 x 48 = 96, and we want a product of 100)
So, when we multiply the numerator (13) by a number a little bit bigger than 2 we get a number a little bit bigger than 26
In other words, 13/48 = (26+)/100
So, this works out to a little bit bigger than 26% . . . Choose B

This technique is further discussed in an article I wrote for BTG: https://www.beatthegmat.com/mba/2013/01/ ... o-percents

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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