A train of passenger cars and a train of freight cars depart from AB at the same time. When the two cars meet, the passenger car has traveled 70% of the total length, and the truck is still 400km away from the midpoint. It takes 20 hours for the bus to complete the journey. How many kilometers does the bus travel per hour?
一列客车和一列货车同时从AB两地对开．当两车相遇时，客车走了全长的70%，货车离中点还有400km．客车行完全程需要20小时，客车每小时行多少千米？
<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(7/10) - (1/2)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">400 / (1/5)</gadget>
<output>2_000</output>

<gadget id="calculator">2_000 / 20</gadget>
<output>100</output>

<result>100</result>