# Fast way to manually calculate percentage increments/decrements

I'm currently studying for the GMAT exam, and I'd like to know if there's a way to manually (and quickly) determine an increment/decrement percent between two numbers.

I know for example, that if I take 5 as my $100%$ reference value and I then compare it against $15$, I find that there was a $200%$ increment with the following formula:

$\text{dPctg}=\frac{15\times100}{5}-100$

However, is there a faster way to get to this fact? Or at least achieve it with less calculations?

Please let me know if there's anything else I may be missing.

Thank you for your time and help!

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 If 5 is 100%, then $15=3\times 5$ is $3\times100\%$.Your increase is then $300-200=$... – J. M. Oct 31 '11 at 21:10

Perhaps $\left(\frac{\textrm{new value}}{\textrm{reference value}} - 1\right) \times 100$ is what you need.
Example: $\left(\frac{15}{5}-1\right)\times 100 = (3-1)\times 100 = 200$.