The store shipped back a batch of stationery, sold 20% of it on the first day, sold 300 sets on the second day, and left 100 sets. How many sets of this batch of stationery are there?
商店运回一批文具，第一天卖出20%，第二天卖出300套，还剩100套，这批文具共有多少套？
<gadget id="calculator">300 + 100</gadget>
<output>400</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">400 / (4/5)</gadget>
<output>500</output>

<result>500</result>