A book is 18 yuan, mother buys 2 books, how much should I get back after paying 50 yuan.
一本书18元，妈妈买2本，付50元应找回多少元．
<gadget id="calculator">18 * 2</gadget>
<output>36</output>

<gadget id="calculator">50 - 36</gadget>
<output>14</output>

<result>14</result>