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