Zhang Yu bought two books yesterday. One is "Naughty Bao Ma Xiaotiao", the unit price is 18 yuan, and the other is "New Mathematics Story", the unit price is 15.5 yuan. He paid the salesperson 50 yuan, how much should he get back?
张雨昨天买了两本书．一本是《淘气包马小跳》，单价18元，一本是《新数学故事》，单价15.5元．他付给售货员50元，应找回多少元？
<gadget id="calculator">18 + 15.5</gadget>
<output>33.5</output>

<gadget id="calculator">50 - 33.5</gadget>
<output>16.5</output>

<result>16.5</result>