Uncle Liu drove to deliver the goods. When he went there, the speed was 42 kilometers per hour. After walking for 6 hours, the car was empty when he came back. It took only 4 hours. How many kilometers per hour did he drive when he came back?
刘叔叔开车送货，去的时候速度是42千米∕时，走了6小时，回来时是空车，只用了4小时，那么回来时每小时行多少千米？
<gadget id="calculator">6 / 4</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">42 * (3/2)</gadget>
<output>63</output>

<result>63</result>