8 people repair 840 meters of roads in 10 days. According to this calculation, how many days will it take for 20 people to repair 4200 meters?
8个人10天修公路840米，照这样算，20人要修4200米，要用多少天．
<gadget id="calculator">840 / 10 / 8</gadget>
<output>21/2 = around 10.5</output>

<gadget id="calculator">20 * (21/2)</gadget>
<output>210</output>

<gadget id="calculator">4_200 / 210</gadget>
<output>20</output>

<result>20</result>