Yangyang takes 20 yuan to buy a notebook, each notebook is priced at a yuan, and he bought 3 copies, how much should he get back.
洋洋带20元去买笔记本，每个笔记本售价a元，他买了3本，应找回多少元．
<gadget id="calculator">20 - 3</gadget>
<output>17</output>

<result>17</result>