Xiao Ming bought a pen and a diary. The unit price of the pen is 12.7 yuan, and the price of the diary is 4.5 yuan. Xiao Ming paid the salesperson 20 yuan, how much should I get back?
小明买了一支钢笔和一本日记本，钢笔的单价是12.7元，日记本的价钱是4.5元．小明付给营业员20元，应找回多少元？
<gadget id="calculator">12.7 + 4.5</gadget>
<output>17.2</output>

<gadget id="calculator">20 - 17.2</gadget>
<output>2.8</output>

<result>2.8</result>