Mark has the option of getting a $300 lens with a 20% discount or a $220 lens.  How much money does he save by buying the cheaper lens?
The discount is worth 300*.2=$<T>multiply(300, 0.2)=60.0
So the lens cost 300-60=$<T>subtract(300, 60)=240
That means he saves 240-220=$<T>subtract(240, 220)=20 by buying the cheaper lens
