A pair of badminton rackets is 30 yuan. Dad bought two badminton rackets and paid 100 yuan. How much should I get back?
一副羽毛球拍30元，爸爸买了两幅羽毛球拍付了100元，应找回多少钱？
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">100 - 60</gadget>
<output>40</output>

<result>40</result>