A train departs from Beijing to Shanghai and travels the whole journey (3/7) in 3 hours. At this time, it is still 40 kilometers away from the midpoint. How many kilometers per hour does the train travel on average?
一列火车从北京开往上海，3小时行了全程的(3/7)，这时距中点还有40千米．这列火车平均每小时行多少千米．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(1/2) - (3/7)</gadget>
<output>1/14 = around 0.071429</output>

<gadget id="calculator">40 / (1/14)</gadget>
<output>560</output>

<gadget id="calculator">(3/7) / 3</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">560 * (1/7)</gadget>
<output>80</output>

<result>80</result>