Zhang Hong bought two books yesterday. The unit price of "Little Doudou by the Window" is 20.00 yuan, and the unit price of "Seed Faith" is 18.3 yuan. She paid the salesperson $50, how much should she get back?
张红昨天买了两本书．一本是《窗边的小豆豆》单价是20.00元，一本是《种子的信念》单价18.3元．她付给售货员50元，应找回多少元？
<gadget id="calculator">20 + 18.3</gadget>
<output>38.3</output>

<gadget id="calculator">50 - 38.3</gadget>
<output>11.7</output>

<result>11.7</result>