A ship travels between A and B. It sails along the current from A to B. The speed of the ship in still water is 20 kilometers per hour. It takes 8 hours to travel from A to B. The time of sailing against the current is 1.5 times the time of sailing along the water. times, find the velocity of the water flow.
一艘轮船往返于A、B两地之间，由A至B是顺水航行，船的静水速度是每小时20千米，由A至B用了8小时，逆水航行时间是顺水航行时间的1.5倍，求水流速度．
<gadget id="calculator">20 * 8 * 1.5</gadget>
<output>240</output>

<gadget id="calculator">20 * 8</gadget>
<output>160</output>

<gadget id="calculator">240 - 160</gadget>
<output>80</output>

<gadget id="calculator">8 * 1.5</gadget>
<output>12</output>

<gadget id="calculator">8 + 12</gadget>
<output>20</output>

<gadget id="calculator">80 / 20</gadget>
<output>4</output>

<result>4</result>