A ship sails between two piers, it takes 3 hours to travel along the current and 4 hours to travel against the current, and the speed of the water is 3 kilometers per hour. What is the distance in kilometers between the two piers?
一艘轮船在两码头间航行，顺水而行需3小时，逆水而行需4小时，水速是每小时3千米．两码头间的距离为多少千米．
<gadget id="calculator">3 * 2 * 3</gadget>
<output>18</output>

<gadget id="calculator">4 - 3</gadget>
<output>1</output>

<gadget id="calculator">18 / 1</gadget>
<output>18</output>

<gadget id="calculator">18 * 4</gadget>
<output>72</output>

<result>72</result>