The bus travels from A to B for 4 hours (2/5). At this time, there are still 39 kilometers away from the midpoint of the journey. What is the speed of the bus?
客车从甲地开往乙地，4小时行全程(2/5)，这时离全程中点还有39千米，求客车的时速？
<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>