A train from Shanghai to Nanjing originally travels 90 kilometers per hour and arrives in 4 hours. After the train speeds up, it travels 120 kilometers per hour. Now how many hours does it take for the train to travel from Nanjing to Shanghai?
一列火车从上海开往南京，原来每小时行90千米，4小时到达，火车大提速后，每小时行120千米，现在这列火车从南京开往上海需几个小时？
<gadget id="calculator">4 / 120</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">90 * (1/30)</gadget>
<output>3</output>

<result>3</result>