Teacher Li went to the bookstore and bought a copy of "Synchronous Learning and Exploration of Mathematics" and a "Workbook of Mathematics". Mathematics Simultaneous Learning and Exploration" is 3.50 yuan cheaper. Teacher Li gave the salesperson 50 yuan, how much should I get back?
李老师到书店买了一本《数学同步学习与探究》和一本《数学练习册》，一本《数学同步学习与探究》的价格为15.60元，一本《数学练习册》比一本《数学同步学习与探究》便宜3.50元．李老师给了售货员50元，应找回多少钱？
<gadget id="calculator">15.6 - 3.5</gadget>
<output>12.1</output>

<gadget id="calculator">50 - 15.6 - 12.1</gadget>
<output>22.3</output>

<result>22.3</result>