Xiaoying bought two books, the prices are 7.5 yuan and 9.8 yuan respectively. She paid 20 yuan, how much should she get back?
小英买了2本书，价格分别是7.5元和9.8元．她付了20元，应找回多少元？
<gadget id="calculator">7.5 + 9.8</gadget>
<output>17.3</output>

<gadget id="calculator">20 - 17.3</gadget>
<output>2.7</output>

<result>2.7</result>