Normally, it takes 20 hours for a ship to travel downstream from point A to point B, and it takes 28 hours to travel upstream from point B to point A. It is the rainy season, and the flow speed of the water is twice the normal speed, so how many hours will it take to go from A to B and back to A?
平时轮船从A地顺流而下到B地要行20小时，从B地逆流上而到A地要行28小时．现正值雨季，水流速度为平时的2倍，那么，从A到B再回A共需多少小时．
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 28</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">(1/20) - (1/28)</gadget>
<output>1/70 = around 0.014286</output>

<gadget id="calculator">(1/70) / 2</gadget>
<output>1/140 = around 0.007143</output>

<gadget id="calculator">(1/20) - (1/140)</gadget>
<output>3/70 = around 0.042857</output>

<gadget id="calculator">(1/140) * 2</gadget>
<output>1/70 = around 0.014286</output>

<gadget id="calculator">(3/70) + (1/70)</gadget>
<output>2/35 = around 0.057143</output>

<gadget id="calculator">1 / (2/35)</gadget>
<output>35/2 = around 17.5</output>

<gadget id="calculator">(1/140) - (1/70)</gadget>
<output>-1/140 = around -0.007143</output>

<gadget id="calculator">(1/28) + (-1/140)</gadget>
<output>1/35 = around 0.028571</output>

<gadget id="calculator">1 / (1/35)</gadget>
<output>35</output>

<gadget id="calculator">(35/2) + 35</gadget>
<output>105/2 = around 52.5</output>

<result>105/2 = around 52.5</result>