A ship sails between two piers A and B. It takes 3 hours to sail along the current and 5 hours to sail against the current. The known current velocity is 4km/h. Find the distance between the two piers.
一船航行于A、B两个码头之间，顺水航行需3h，逆水航行需5h，已知水流速度是4km/h，求这两个码头之间的距离．
<gadget id="calculator">4 + 4</gadget>
<output>8</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/3) - (1/5)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">8 / (2/15)</gadget>
<output>60</output>

<result>60</result>