Xiaohong used 20 yuan to buy stationery, 6.7 yuan to buy a pen, and 9.8 yuan to buy a stationery box. How much money is left?
小红用20元钱买文具，买钢笔用去6.7元，买文具盒用去9.8元，还剩下多少钱？
<gadget id="calculator">6.7 + 9.8</gadget>
<output>16.5</output>

<gadget id="calculator">20 - 16.5</gadget>
<output>3.5</output>

<result>3.5</result>