A car travels from A to B at 60 kilometers per hour and returns at 90 kilometers per hour. Find the average round trip speed of the car.
一辆汽车从甲地开往乙地，每小时行60千米，回来时，每小时行90千米．求这辆汽车往返的平均速度．
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / 90</gadget>
<output>1/90 = around 0.011111</output>

<gadget id="calculator">(1/60) + (1/90)</gadget>
<output>1/36 = around 0.027778</output>

<gadget id="calculator">2 / (1/36)</gadget>
<output>72</output>

<result>72</result>