The distance between A and B is 540 kilometers. A car travels from A to B and then returns. It travels 90 kilometers per hour when going there and 60 kilometers per hour when it returns. Find the average speed of the car going back and forth?
甲、乙两地相距540千米，一辆汽车从甲地到乙地，然后返回，去时每小时行90千米，回来每小时行60千米，求该车往返的平均速度？
<gadget id="calculator">540 / 90</gadget>
<output>6</output>

<gadget id="calculator">540 / 60</gadget>
<output>9</output>

<gadget id="calculator">6 + 9</gadget>
<output>15</output>

<gadget id="calculator">2 / 15</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">540 * (2/15)</gadget>
<output>72</output>

<result>72</result>