The distance between two ports A and B is 90 kilometers, and it takes 20 hours for a passenger ship to go back and forth between the two ports. It is known that the downstream speed is three times the upstream speed. The speed of a cargo ship in still water is twice that of a passenger ship in still water, so how many hours does it take for a cargo ship to go back and forth between the two ports?
A、B两港相距90千米，客船往返两港需20小时，已知顺流速度是逆流速度的3倍．货船的静水速度是客船静水速度的两倍，那么货船往返两港需要多少小时？
<gadget id="calculator">6 + 18</gadget>
<output>24</output>

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">18 - 6</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">24 + 6</gadget>
<output>30</output>

<gadget id="calculator">90 / 30</gadget>
<output>3</output>

<gadget id="calculator">24 - 6</gadget>
<output>18</output>

<gadget id="calculator">90 / 18</gadget>
<output>5</output>

<gadget id="calculator">3 + 5</gadget>
<output>8</output>

<result>8</result>