A car departs from city A to city B, travels 60 kilometers per hour when going there, arrives at city B at 3 o'clock, and travels 90 kilometers per hour when returning, what is the average speed of this car going back and forth?
一辆车从甲城出发去乙城，去时每时行60千米，用3时到达乙地，回来时每时行90千米，这辆车来回的平均速度是多少？
<gadget id="calculator">3 / 90</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">60 * (1/30)</gadget>
<output>2</output>

<gadget id="calculator">3 + 2</gadget>
<output>5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">60 * 3 * (2/5)</gadget>
<output>72</output>

<result>72</result>