A lifeboat has to travel 840 kilometers from the port to the accident site, and the speed of the ship is 20 kilometers per hour. A helicopter on board can fly 220 kilometers per hour. The plane takes off midway and arrives at the accident site in advance. It took 10 hours from the time the ship left the port to when the plane arrived at the accident site. How long did it take for the plane to take off after the ship left the port?
一只救生船从港口开到出事地点要行840千米，船速每小时20千米，船上一架直升飞机，每小时可飞行220千米，中途飞机起飞，提前赶到出事地点，这样从船离港口到飞机到达出事地点一共用了10小时，飞机在船离港口后多长时间起飞？
<gadget id="calculator">20 * 10</gadget>
<output>200</output>

<gadget id="calculator">840 - 200</gadget>
<output>640</output>

<gadget id="calculator">220 - 20</gadget>
<output>200</output>

<gadget id="calculator">640 / 200</gadget>
<output>16/5 = around 3.2</output>

<gadget id="calculator">10 - (16/5)</gadget>
<output>34/5 = around 6.8</output>

<result>34/5 = around 6.8</result>