The passenger car drives from place A to another place, and travels the whole journey (2/5) in 4 hours. At this time, it is still 39 kilometers away from the midpoint of the journey. How many kilometers does this car travel on average per hour?
客车从甲地开往已地，4小时行了全程的(2/5)，这时离全程的中点还有39千米，这辆汽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">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">39 / (1/10)</gadget>
<output>390</output>

<gadget id="calculator">390 * (2/5)</gadget>
<output>156</output>

<gadget id="calculator">156 / 4</gadget>
<output>39</output>

<result>39</result>