and if something more difficult - 18% or 27%
Multiply by 0.18 or 0.27? :) This is if you need to know 18% or 27% of the number. If you take this value, then from the unit to take 0.18 or 0.27, then multiply. I do not believe that it is difficult to make a calculation in mind (1 - 0.18). Although of course I understand that when there are many such calculations, it is necessary to avoid a human error, it is better to use some other method, but for one-time computing this scheme will go. Again, I repeat that I trally do not know how to use the function of% on the calculators, so I can not even tell.