It takes 12 hours for a small ship to sail 120 kilometers downstream and 90 kilometers upstream; at this speed, it takes 8 hours to sail 100 kilometers downstream and 45 kilometers upstream. 30 kilometers, how many hours does it take for this small ship to go back and forth?
一艘小轮船顺流航行120千米，逆流航行90千米需要12小时；按这样的速度顺流航行100千米，逆流航行45千米需要8小时，如果一静水湖中有两个码头相距30千米，这艘小轮船往返一趟需要多少小时？
<gadget id="calculator">200 - 120</gadget>
<output>80</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

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

<gadget id="calculator">30 / 20</gadget>
<output>3/2 = around 1.5</output>

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

<gadget id="calculator">90 / 6</gadget>
<output>15</output>

<gadget id="calculator">30 / 15</gadget>
<output>2</output>

<gadget id="calculator">(3/2) + 2</gadget>
<output>7/2 = around 3.5</output>

<result>7/2 = around 3.5</result>