A truck departs from the county town to deliver textbooks to Hot Spring Township, travels 42 kilometers per hour, and arrives in 3 hours. When it returns, it is empty and travels 21 kilometers more per hour than when it went there. How long did it take for the truck to return to the county seat?
一辆货车从县城出发，送教科书到温泉乡，每小时行42千米，3小时到达，返回时由于是空车，每小时比去时多行21千米．这辆货车返回县城时用了多少时间？
<gadget id="calculator">42 + 21</gadget>
<output>63</output>

<gadget id="calculator">3 / 63</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">42 * (1/21)</gadget>
<output>2</output>

<result>2</result>