Tonya has $150.00 on her credit card.  If she leaves any balance on her card at the end of the month, she is charged 20% interest.  If she makes a $50.00 payment on her card, what will be the new balance?
Her card has a $150.00 balance and she makes a $50.00 payment so the new balance is 150-50 = $
<gadget id="calculator">150-50</gadget>
<output>100</output>
100.00.
She didn't pay it off so she is charged 20% interest on her $100.00 balance so the interest is .20*100 = $20.00.
Her balance was $100.00 and she was charged $20.00 in interest so her new balance is 100+20 = $
<gadget id="calculator">100+20</gadget>
<output>120</output>
120.00.

<result>120</result>