There are two terminals A and B along the Yangtze River. It is known that a passenger ship travels 500 kilometers per day from A to B, and 400 kilometers per day from B to A. , then the distance between the two piers is how many kilometers.
长江沿岸有A、B两码头，已知客船从A到B航行每天行500千米，从B到A航行每天行400千米，如果客船在A、B两码头间往返航行5次共用18天，那么两码头间的距离是多少千米．
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">18 / 5 / 9</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) * 4</gadget>
<output>8/5 = around 1.6</output>

<gadget id="calculator">500 * (8/5)</gadget>
<output>800</output>

<result>800</result>