If it takes 2 hours for a boat to sail 40 kilometers downstream, how many hours will it take for the boat to sail 40 kilometers upstream if the speed of the current is 2 kilometers per hour?
一条船顺流航行40千米用2小时，如果水流速度为每小时2千米，那么这条船逆流航行40千米用多少小时．
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 - 2 - 2</gadget>
<output>16</output>

<gadget id="calculator">40 / 16</gadget>
<output>5/2 = around 2.5</output>

<result>5/2 = around 2.5</result>