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!
Best Answer
Perhaps $\left(\frac{\textrm{new value}}{\textrm{reference value}} - 1\right) \times 100$ is what you need.
Example: $\left(\frac{15}{5}-1\right)\times 100 = (3-1)\times 100 = 200$.
This has the benefit of giving you a fast way to "ballpark" what you're looking for. So if you had, say, 397 as the new value, and 53 as the reference value, then you know that 50 is a little less than 53, so 8*50 is a little less than 8*53. Combine that with the fact that 397 is a little less than 400 = 8*50, and consequently your increase will be a little less than (8-1)*100 = 700.