The fuel carried by an aircraft can be used up to 9 hours. When the aircraft goes with the wind, the flight speed is 1500km/h, and when it returns to the headwind, the flight speed is 1200km/h. How many kilometers does the aircraft fly before it needs to fly back?
一架飞机所带燃料最多可以用9h，飞机去时顺风，飞行速度为1500km/h，返回时逆风，飞行速度为1200km/h，这架飞机最多飞出多少km，就需要往回飞？
<gadget id="calculator">1 / 1_500</gadget>
<output>1/1_500 = around 0.000667</output>

<gadget id="calculator">1 / 1_200</gadget>
<output>1/1_200 = around 0.000833</output>

<gadget id="calculator">(1/1_500) + (1/1_200)</gadget>
<output>3/2_000 = around 0.0015</output>

<gadget id="calculator">9 / (3/2_000)</gadget>
<output>6_000</output>

<result>6_000</result>