The unit price of each exercise book is 0.45 yuan, Xiaoxiao pays the salesperson aunt 20 yuan, and buys 28 exercise books, how much should I get back?
每本练习本单价0.45元，笑笑付给售货员阿姨20元，买28本练习本，应找回多少元？
<gadget id="calculator">0.45 * 28</gadget>
<output>12.6</output>

<gadget id="calculator">20 - 12.6</gadget>
<output>7.4</output>

<result>7.4</result>