Mr. Zhang bought a pen at a unit price of 15.87 yuan and a book at 15.42 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15.87元的钢笔和一本15.42元的书，他付了50元，应找回多少元？
<gadget id="calculator">50 - 15.87 - 15.42</gadget>
<output>18.71</output>

<result>18.71</result>