It is 140 kilometers from Dehua to Quanzhou. It takes 3 hours for a car to go there and 4 hours to return along the original road. Find the average speed of this car going back and forth?
从德化到泉州有140千米，一辆汽车去时用了3小时，沿原路返回用了4小时，求这辆汽车往返的平均速度？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">140 * (2/7)</gadget>
<output>40</output>

<result>40</result>