[zork]

One of the best things when it comes to percentages is the "Dreisatz" (rule of proportion).

http://en.wikipedia.org/wiki/Rule_of_proportion

So easy example. You have 100$. That is 100% of your money. You want so substract 20% of it. So you can write down:

100$     x
----  =  ---
100%     80%

x is the value you want to calculate and it is 100%-20% = 80% of your money.

Now all you have to do is to make the value for x come out. (Having x stand on one side)

To make that possible you multiplay both side of the equation with 80%. By doing that the 80% on the right side will fade because 80% / 80% is 1.

100$*80%      x*80%
--------   =  ---
100%          80%

100$*80%      x*1
--------   =  ---
100%          1

100$*80%      
--------   =  x
100%

Result:

x = 100$*80%/100%

x = 80$

The cool thing about that is that even the suffixes come out accordingly. The % delete themselve only the $ stays. Btw. 8000/100 is the same as 100*0.8. Doesn't matter.