From A to B, the express train travels 84 kilometers per hour and arrives in 3 hours. The speed of the slow train is only 5/7 of that of the express train. How long does it take for the slow train to arrive?
从甲地到乙地，快车每小时行84千米，3小时到达，慢车的速度只有快车的(5/7)，慢车几小时到达？
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">84 * (5/7)</gadget>
<output>60</output>

<gadget id="calculator">3 / 60</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">84 * (1/20)</gadget>
<output>21/5 = around 4.2</output>

<result>21/5 = around 4.2</result>