passenger car drives from point A to point B, and it travels the whole distance (2/5) in 4 hours. At this time, it is still 39 kilometers away from the midpoint. How many kilometers does this passenger car drive on average per hour?
一辆客车从A地开往B地，4小时行驶了全程的(2/5)，这时离中点还有39千米，这辆客车平均每小时行驶多少千米．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(1/2) - (2/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">4 / (2/5)</gadget>
<output>10</output>

<gadget id="calculator">39 / (1/10) / 10</gadget>
<output>39</output>

<result>39</result>