There is a batch of books in the library, 23% of which are out on the first day and 35% on the second day, leaving 840 books. How many books are there in this batch?
图书馆有一批书，第一天外借23%，第二天外借35%，还剩840本书，这批书共有多少本？
<gadget id="calculator">23 / 100</gadget>
<output>23/100 = around 0.23</output>

<gadget id="calculator">35 / 100</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">1 - (23/100) - (7/20)</gadget>
<output>21/50 = around 0.42</output>

<gadget id="calculator">840 / (21/50)</gadget>
<output>2_000</output>

<result>2_000</result>