SOLUTION: The current of a stream moves at a speed of 5km/hr. The boat travels 31 km upstream and 31 km downstream in a total time of 11 hours. What is the speed of the boat in still water?

Question 548514: The current of a stream moves at a speed of 5km/hr. The boat travels 31 km upstream and 31 km downstream in a total time of 11 hours. What is the speed of the boat in still water?
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
The current of a stream moves at a speed of 5km/hr. The boat travels 31 km upstream and 31 km downstream in a total time of 11 hours. What is the speed of the boat in still water?
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Upstream DATA:
distance = 31 km ; rate = b-5 km/hr ; time = d/r = 31/(b-5)hrs
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Downstream DATA:
distance = 31 km ; rate = b+5 km/hr ; time = d/r = 31/(b+5) hrs
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Equation:
31/(b-5) + 31/(b+5) = 11
31(b+5) + 31(b-5) = 11(b^2-25)
62b = 11b^2-275
11b^2 - 62b -275 = 0
positive solution: b = 8.58 km/hr (speed of boat in still water)
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Cheers,
Stan H.
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