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A motorboat is capable of traveling at a speed of 12 miles per hour in still water. On a particular​ day, it took

15 minutes longer to travel a distance of 4 miles upstream than it took to travel the same distance downstream. What was the rate of current in the stream on that​ day?

 Feb 3, 2020
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On an ordinary day, the boat can cover the distance of 4 miles in:

 

4 /12 =1/3 of an hour, or 1/3 x 60 = 20 minutes.

But, on some particular day, it took the boat: 20 + 15 = 35 minutes to travel that 4 miles.

So: 4 / x =35/60

x =4 / (35/60)

x =6  6/7 mph - the speed of the boat on that particular day.

12 - 6  6/7 = 5 1/7 mph - the rate of the current on that day.

 Feb 3, 2020

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