# Percentage change

case math
New answer on Jul 04, 2021
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Some people are great with percentages, they can just look at some number change - for eg if it changed from 550 to 900, and just say it changed by x% without putting pen to paper (especially during exhibit reading). Are there any tricks to get better at this without having to write it down and calculate the change?

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On this specific one, here my appraoch:

Instead of trying to calculate in percentages, try to calculate in fractions, for example:

percentage = (900 - 550) / 550 = 350 / 550

Then you can range the result by looking for fitting numbers, e.g:

350 / 700 = 1/2 = 50%, so you have a lower bound of 50%

350 / 500 = 7/10 = 70%, so you have an upper bound of 70%

550 is closer to 500 than to 700, so the result would be somerhwere in the 65% range. There is typically no need to be more precise than this.

To practice this, I would recommend the math drill tool on this website.

Hi there,

Here, practice honestly makes perfect (there are plenty of resources out there for rote practice).

That said, the main way to get better at this is to learn the major fractions (3/4 = 75%, 2/3 = 67%, etc.), then look at what the number is close to.

For example, 550 out of 900 is close to 500/1000 which is 50%, but we know we've rounded down the numerator and up the denominator so it must be closer to 60%