SOLUTION: a boat travels 100 miles downstream in the same length of time it travels upstream. if the boats speed in still water is 30 mph, find the speed of the current river.

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Question 138377: a boat travels 100 miles downstream in the same length of time it travels upstream. if the boats speed in still water is 30 mph, find the speed of the current river.
Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
the speed of the current must be zero.
Let the current speed be "c":
Upstream Data:
distance = 100 miles ; rate = 30-c mph ; Time = d/r = 100/(30-c) hrs
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Downstream DATA:
distance = 100 miles ; rate = 30+c mph ; Time = d/r = 100/30+c hrs
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EQUATION:
time up = time down
100/(30-c) = 100/(30+c)
30-c = 30+c
2c = 0
c = zero
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There is no upsteam or downstream on this particular river.
It must be a lake.
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Cheers,
Stan H.