A workshop started to transfer one person every day from April 2, 2011. It is known that each person produces one product per day. The workshop produced a total of 840 products from April 1 to April 21. How many workers did the workshop have? ．
某车间从2011年4月2日开始每天调入一人，已知每人每天生产1件产品，该车间从4月1日至4月21日共生产840件产品，该车间原有工人多少名．
<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>