somebody makes widgets for living, he gets paid $100 a day for his good work; but for every widget he made, if it does NOT pass quality inspection, he must pay $2 penalty because he then wasted raw material and machine time, so he got paid by the following formula:
Pay = 100 - 2 * # of bad widgets
let's further assume on average he makes 5 bad widgets a day no matter how carefully he is (since he is short of basic math skill), so his average daily wage is:
100 - 2 * 5 = 100 - 10 = $90
Last month, he decided to learn some basic math, which helps him reducing his bad widgets by 2 everyday on average, now his new daily wage is:
100 - 2 * (5 - 2) = 100 - 6 = 94
so, one can see (-2) * (-2) = 4, that's exactly his daily wage increase.
不是要咱们做小学作业吧 ：）
