Ding Ding bought three books at prices of 12.50 yuan, 13.40 yuan, and 10.20 yuan respectively. He paid the salesperson 50 yuan. How much did he get back?
丁丁买了三本书，价格分别为12.50元、13.40元、10.20元，他付给营业员50元，找回多少元？
<gadget id="calculator">12.5 + 13.4 + 10.2</gadget>
<output>36.1</output>

<gadget id="calculator">50 - 36.1</gadget>
<output>13.9</output>

<result>13.9</result>