If you are adjusting shutter speeds, you multiply.

If you are adjusting apertures, you add the base 2 logs of them to get the change in stops.

The base 2 logs are the number you "raise" 2 to to get your factor. And they are the stop equivalent of the factor.

Here is a calculator: http://www.miniwebtool.com/log-base-2-calculator/

so for your factor numbers 2.15 and 2.5 the result is 1.1 + 1.3 = 2.4 stops (after rounding off).