A ship travels 30 kilometers per hour along the current from Port A to Port B, and travels 20 kilometers per hour against the current from Port B to Port A. A round trip takes 9 hours. How many kilometers is the distance between A and B ports?
轮船从甲港列乙港顺水每小时时行30千米，从，乙港到甲港逆水每小时行20千米．往返一次共9个小时，甲、乙两港口相距多少千米？
<gadget id="calculator">30 * 18</gadget>
<output>540</output>

<result>540</result>