Uncle Wang drove from place A to place B. He traveled 40km per hour when he went there, and 60km per hour when he returned. It took him 6 hours to go back and forth. How long did it take him to return? (Solve with ratio)
王叔开车从甲地到乙地，去时每小时行40km，返回时每小时行60km，往返共用了6小时，返回时用了多长时间？(用比例解)
<gadget id="calculator">6 - 3.6</gadget>
<output>2.4</output>

<result>2.4</result>