Uncle Zhang drove to other places to study at a speed of 54 km/h and arrived in 4 hours. When Uncle Zhang returned, he traveled 72 km/h. How many hours did it take to return?
张叔叔开车去外地学习，速度是54千米/小时，4小时到达，张叔叔返回时每小时行72千米．问返回时用了几小时？
<gadget id="calculator">4 / 72</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">54 * (1/18)</gadget>
<output>3</output>

<result>3</result>