Xiaohua bought a pencil case and an exercise book at the stationery store. The pencil case was 7.5 yuan, and the price of the pencil case was 2.8 yuan more expensive than the exercise book. Xiaohua paid 20 yuan, how much should I get back?
小华在文具店买了一个文具盒和一本练习本，文具盒7.5元，文具盒的价钱比练习本贵2.8元．小华付出20元，应找回多少钱？
<gadget id="calculator">7.5 - 2.8</gadget>
<output>4.7</output>

<gadget id="calculator">20 - 7.5 - 4.7</gadget>
<output>7.8</output>

<result>7.8</result>