There was a group of workers in a workshop, and one person was transferred every day from September 2. If each person produced one product per day, the workshop produced a total of 840 products from September 1 to September 21. How many workers were there in the workshop?
某车间原有一批工人，从9月2日开始每天调入1人，如果每人每天生产一件产品，该车间从9月1日至9月21日共生产840件产品．该车间原有多少名工人？
<gadget id="calculator">21 / 2</gadget>
<output>21/2 = around 10.5</output>

<gadget id="calculator">21 * (21/2)</gadget>
<output>441/2 = around 220.5</output>

<gadget id="calculator">840 - (441/2)</gadget>
<output>1_239/2 = around 619.5</output>

<gadget id="calculator">(21/2) + (1_239/2)</gadget>
<output>630</output>

<gadget id="calculator">(21/2) + (21/2)</gadget>
<output>21</output>

<gadget id="calculator">630 / 21</gadget>
<output>30</output>

<result>30</result>