The car drove from Nanjing to Shanghai. It used to travel 60 kilometers per hour, and it took 6 hours to get there. After the car returns, it is 90 kilometers per hour. How long does it take to return to Shanghai from Nanjing?
汽车从南京开往上海，原来每小时行驶60千米，要6小时才能到达．汽车返回后，每小时90千米，现在从南京返回上海需要多长时间？
<gadget id="calculator">6 / 90</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">60 * (1/15)</gadget>
<output>4</output>

<result>4</result>