The distance between A and B is 3240 kilometers. An airplane performs a flight mission from A to B, and then returns from B to A. When flying out, it flies 810 kilometers per hour, and when it returns, it flies 540 kilometers per hour. How many kilometers per hour does the plane fly round-trip on average?
甲、乙两地相距3240千米，一架飞机从甲地到乙地执行飞行任务，又从乙地返回甲地．飞出时每小时飞行810千米，返回时每小时飞行540千米．这架飞机往返平均每小时飞行多少千米？
<gadget id="calculator">3_240 / 810</gadget>
<output>4</output>

<gadget id="calculator">3_240 / 540</gadget>
<output>6</output>

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

<gadget id="calculator">2 / 10</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">3_240 * (1/5)</gadget>
<output>648</output>

<result>648</result>