The supermarket bought a box of bananas weighing 20 kilograms in the wholesale market and spent 50 yuan. Then sell it at 3.5 yuan per kilogram. After a box of bananas is sold out, how much money do you make?
超市在批发市场进了一箱重20千克的香蕉，花了50元．然后以每千克3.5元的出售，一箱香蕉卖完后，赚了多少钱？
<gadget id="calculator">3.5 * 20</gadget>
<output>70</output>

<gadget id="calculator">70 - 50</gadget>
<output>20</output>

<result>20</result>