Xinhua Bookstore shipped a batch of children's books, sold 1,800 books on the first day, sold (1/9) more than the first day on the second day, and sold out the remaining total (3/7) on the third day. How many children's books are there in this batch?
新华书店运来一批儿童读物，第1天卖出1800本，第2天比第1天多卖(1/9)，余下总数的(3/7)第3天全部卖完．这批儿童读物共有多少本？
<gadget id="calculator">2_000 + 1_800</gadget>
<output>3_800</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">3_800 / (4/7)</gadget>
<output>6_650</output>

<result>6_650</result>