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Percentages

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by RobertP » Fri Jan 22, 2016 12:07 pm
Hello,

This is my first time posting so I am not entirely sure that this is the right place for this sort of question.

I would like someones opinion on the best way to figure out complicated percentages.

Right now I know of two techniques (maybe three) that I use, but they take to long. For example:

What percentage is 72 of 630? The techniques I would use would be:

1) Find 10%, 5%, 1% of 630 and mix and match to find what is the percentage;

2) Do the long division. (I personally do not like this method because finding multiples of 630 to the remainders is usually a pain); and

3) Fraction them, find a common divisor and do the long division or percentage. (In the past this has proven to be both useful and sometimes a waste of time).

Anyone got any opinions on the matter?

Thank you!
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Source: — Problem Solving |

by Matt@VeritasPrep » Fri Jan 22, 2016 4:26 pm
You can express a percentage as (p/100). So your question, translated to algebra, looks like this:

What percentage of 630 is 72?

(p/100) * 630 = 72

p * 630 = 72 * 100

p * 63 = 72 * 10

p * 7 = 80

p = 80/7

So it's (80/7 / 100), or (11 + 3/7)%.

Another option is just to use a ratio:

72/630 = p/100

Then solve from there.

On the GMAT, your answer would simplify to something friendlier than 8/70.
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by [email protected] » Fri Jan 22, 2016 7:19 pm
Hi RobertP,

Beyond the methods that you've described, you should look at the 'context' of the question that you're trying to answer. The Quant section of the GMAT will very rarely offer up a question that can only be solved by complex calculations. The question might ask for an approximation (hinting that you should NOT do a lot of math) or the answer choices might be 'spaced out' so that a bit of logic can be used to eliminate the wrong answers (they would likely be 'too big' or 'too small' to be correct). You might be dealing with a DS prompt that has an algebraic 'shortcut' or Number Property rule that you can take advantage of to get to the correct answer faster.

If you have specific prompts that are causing you any trouble, then you should post them here (in the proper sub-Forums) and you'll likely receive plenty of advice on each.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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