There are 840 seats downstairs in the theater, which is 21 times that of the seats upstairs. How many seats does this theater have?
剧院楼下有座位840个，是楼上座位的21倍，这个剧院一共有多少个座位？
<gadget id="calculator">840 / 21</gadget>
<output>40</output>

<gadget id="calculator">40 + 840</gadget>
<output>880</output>

<result>880</result>