A ship sails between two piers. The speed of the current is 3 km/h. It takes 2 hours to sail along the current and 3 hours to sail against the current. Find the distance between the two piers.
一艘船在两个码头之间航行，水流的速度是3千米/时，顺水航行需要2小时，逆水航行需要3小时，求两码头之间的距离．
<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">9 + 6</gadget>
<output>15</output>

<gadget id="calculator">3 - 2</gadget>
<output>1</output>

<gadget id="calculator">15 / 1</gadget>
<output>15</output>

<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<gadget id="calculator">18 * 2</gadget>
<output>36</output>

<result>36</result>