The maximum safe amount of caffeine you can consume per day is 500 mg. If each energy drink has 120 mg of caffeine and Brandy drinks 4 of them, how much more caffeine can she safely consume that day?
First find the total amount of caffeine Brandy already drank: 120 mg/drink * 4 drinks = 
<gadget id="calculator">120*4</gadget>
<output>480</output>
480 mg.
Then subtract that amount from the amount it's safe to consume daily to find the remaining amount Brandy can consume: 500 mg - 480 mg = 
<gadget id="calculator">500-480</gadget>
<output>20</output>
20 mg.

<result>20</result>