The distance between A and B is 140 kilometers. It takes 2.5 hours for a car to travel from A to B, and it takes 1 hour less to return. What is the average speed of this car going back and forth?
甲乙两地相距140千米，一辆汽车从甲地到乙地用2.5小时，返回时少用1小时，这辆汽车往返的平均速度是多少．
<gadget id="calculator">2.5 - 1</gadget>
<output>1.5</output>

<gadget id="calculator">2.5 + 1.5</gadget>
<output>4</output>

<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">140 * (1/2)</gadget>
<output>70</output>

<result>70</result>