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!