River A is a tributary of River B. The flow rate of River A is 3 kilometers per hour, and the flow rate of River B is 2 kilometers per hour. A boat sailed along River A for 6 hours and traveled 126 kilometers to reach River B. , 64 kilometers will be sailed against the current on the B River. How many hours will this ship sail?
A河是B河的支流，A河水的流速为每小时3千米，B河水的流速是每小时2千米，一艘船沿A河顺水航行了6小时，行了126千米到达B河，在B河还要逆水航行64千米，这艘船还要航行几小时？
<gadget id="calculator">126 / 6</gadget>
<output>21</output>

<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<gadget id="calculator">18 - 2</gadget>
<output>16</output>

<gadget id="calculator">64 / 16</gadget>
<output>4</output>

<result>4</result>