A ship goes back and forth between A and B. From A to B is sailing with the current, and from B to A is sailing against the current. It is known that the speed of the boat in still water is 20 dry meters per hour. It takes 6 hours to go from A to B, and the time taken to go from B to A is 1.5 times the time taken from A to B. Find the speed of the water.
一艘轮船往返于A、B两地之间，由A到B是顺水航行，由B到A是逆水航行．已知船在静水中的速度是每小时20干米，由A到B用了6小时，由B到A所用时间是由A到B所用时间的1.5倍，求水流速度．
<gadget id="calculator">20 * 6 * 1.5</gadget>
<output>180</output>

<gadget id="calculator">20 * 6</gadget>
<output>120</output>

<gadget id="calculator">180 - 120</gadget>
<output>60</output>

<gadget id="calculator">6 * 1.5</gadget>
<output>9</output>

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

<gadget id="calculator">60 / 15</gadget>
<output>4</output>

<result>4</result>