The tires of a car can travel 2,000 kilometers on the front wheels and 6,000 kilometers on the rear wheels. If the front and rear wheels are allowed to be replaced once in the middle, how many kilometers can the 4 tires travel at most?
汽车的轮胎，放在前轮可以行使2000千米，放在后轮可以行使6000千米，如果允许中途将前轮后轮调换一次，4只轮胎最多可行使多少千米．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 2_000</gadget>
<output>1/2_000 = around 0.0005</output>

<gadget id="calculator">1 / 6_000</gadget>
<output>1/6_000 = around 0.000167</output>

<gadget id="calculator">(1/2_000) + (1/6_000)</gadget>
<output>1/1_500 = around 0.000667</output>

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

<result>3_000</result>