An airplane travels between cities A and B, which are 1620 kilometers apart, at 810 kilometers per hour when going there, and 540 kilometers per hour when returning. How many kilometers per hour does the plane fly round-trip on average?
一架飞机往返相距1620千米的甲、乙两城，去时每小时行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">1_620 / 810</gadget>
<output>2</output>

<gadget id="calculator">1_620 / 540</gadget>
<output>3</output>

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

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

<gadget id="calculator">1_620 * (2/5)</gadget>
<output>648</output>

<result>648</result>