A ship sails from port A to port B and sails along the current for 30 kilometers per hour. When it returns to port A, it takes 7 hours to sail against the current. The known water speed is 3 kilometers per hour. How many kilometers is the distance between ports A and B? ?
一艘轮船从A港列B港，顺水航行每小时30千米，返回A港时逆水航行用了7小时，已知水速是每小时3千米，问A、B两港相距多少千米？
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">30 - 6</gadget>
<output>24</output>

<gadget id="calculator">24 * 7</gadget>
<output>168</output>

<result>168</result>