The road between the two market towns is downhill except for uphill. There is no horizontal road section. The speed of the bus uphill is kept at 15 kilometers, and the speed of downhill is kept at 30 kilometers per hour. It is known that the bus travels between the two places Once, you need to drive on the road for 4 hours, and find the distance between the two places.
两个集镇之间的公路除了上坡就是下坡，没有水平路段，客车上坡的速度保持为15千米，下坡的速度保持为每小时30千米，现知道客车在两地之间往返一次，需在路上行驶4个小时，求两地之间的距离．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/15) + (1/30)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">4 / (1/10)</gadget>
<output>40</output>

<result>40</result>