Mr. Zhang bought a pen with a unit price of 15.8 yuan and a book with a unit price of 15.4 yuan in a stationery store. He paid 50 yuan, how much should he get back?
张老师在文具店里买了一支单价15.8元的钢笔和一本15.4元的书，他付了50元，应找回多少元？
<gadget id="calculator">15.8 + 15.4</gadget>
<output>31.2</output>

<gadget id="calculator">50 - 31.2</gadget>
<output>18.8</output>

<result>18.8</result>