A car starts from City A to City B, travels 60 kilometers per hour when going there, travels for 6 hours, and travels 90 kilometers per hour when it returns. What is the average speed of the car back and forth in kilometers?
一辆车从甲城出发到乙城，去时每小时行驶60千米，行驶了6个小时，回来每小时行90千米．这辆车来回的平均速度是多少千米？
<gadget id="calculator">60 * 6 * 2</gadget>
<output>720</output>

<gadget id="calculator">6 / 90</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">60 * (1/15)</gadget>
<output>4</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<gadget id="calculator">720 / 10</gadget>
<output>72</output>

<result>72</result>