There are 300 exercise books in the stationery store, after selling some, there are 4 packs left, each pack of 20 books, how many books are sold.
文具店有300本练习本，卖出一些后，还剩4包，每包20本，卖出多少本．
<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">300 - 80</gadget>
<output>220</output>

<result>220</result>