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A boat travels 14 km/h in still water. If this boat goes upstream in a river flowing at a rate of 4 km/h, for 3 km, and then returns back downstream to its starting point, what is the average speed for the whole trip in km/h?

 Oct 26, 2021
 #1
avatar+115335 
+2

It goes upstream at 10km/hour for 3 km so that takes  0.3 of an hour

 

It goes downstream tat  18km/hour for 3km so that takes  1/6 of an hour

 

Total distance = 6km    total time = (18+10)minutes = 28/60 hour

 

ave speed =   \(6\div \frac{28}{60}=12\frac{6}{7}\;km/hour\)

 

Sorry guest answerers, I was correcting my answerer without seeing that you had already done so.

 

I did make a very careless error and I thank you for correcting it.

 Oct 26, 2021
edited by Melody  Oct 26, 2021
edited by Melody  Oct 26, 2021
edited by Melody  Oct 26, 2021
 #2
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+2

14 - 4 ==10 km/h - its speed upstream

 

3 km / 10 km/h ==3/10 x 60 ==18 minutes going upstream

 

14 + 4 ==18 km/h - its speed downstream

 

3 km/18 km/h ==1/6 x 60 ==10 minutes going downstream

 

Average speed ==total distance travelled / time

                            == [3 * 2] / [18 + 10] / 60

                            ==  6 / [28 / 60]

                            == 6  * 60 / 28 ==360 / 28 ==12 6/7 km/h

 Oct 26, 2021
 #3
avatar
+2

It goes upstream at 10km/hour for 3 km so that takes  0.3 of an hour

 

It goes downstream tat  18km/hour for 3km so that takes  1/6 of an hour

 

Total distance = 6km    total time = (18+10)minutes = 28/60 = 0.4667 hour

 

ave speed = 6/0.4667 = 12.86 km/hour

 Oct 26, 2021

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