simple math ?

actually, simple math isn't 'simple'. 
Things have to work, they must function. 
Can't always go flying around in the aether ...

  what's a 15% tip?  a 20% tip less 1/2 the 10% tip.
that's only 3 steps.
  
  what does 40% off mean?
  150% ?

As long as you can't do the basics to see what they're doing to you, you are trapped by 'them'. 
   Trick you into buying into 'economic theory', 'maximize your profit on the market', but you don't SEE that the cost of toilet paper has gone up about 35%.  LOL, At this rate, we need to get the newspapers started again.