A new batch of children's books arrived at Xinhua Bookstore, sold 1/5 of the total on the first day, and 40% of the total on the second day, leaving 400 books. How many children's books are there in total?
新华书店新到一批儿童读物，第一天卖出总数的(1/5)，第二天卖出总数的40%，还余下400本，这批儿童读物一共有多少本？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(1/5) + (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">400 / (2/5)</gadget>
<output>1_000</output>

<result>1_000</result>