[Seerah]

I have never, ever seen that method using proportions before, Zork.

(At least in America) we teach that this is how to use proportions to solve percent problems.

 percent     portion 
--------- = ---------
   100        whole

so, let's say you want 80% of 200:

 80       x  
----- = -----
 100     200

Then you cross-multiply

80 * 200 = 100 * x

divide by 100 on both sides to solve for x

( 80 * 200 ) / 100 = 100x / 100
160 = x

(I'm a math teacher.)