Uncle Zhang bought two books in the bookstore. One copy is 28.5 yuan, and the other is 14.4 yuan. He pays 50 yuan, how much should he get back?
张叔叔在书店买了两本书．一本28.5元，另一本14.4元．他付出50元，应找回多少元？
<gadget id="calculator">28.5 + 14.4</gadget>
<output>42.9</output>

<gadget id="calculator">50 - 42.9</gadget>
<output>7.1</output>

<result>7.1</result>