SOLUTION: A boat, which moves at 18 miles per hour in water without a current, goes 80 miles upstream and 80 miles back again in 9 hours. Find the speed of the current to the nearest tenth.

Question 706308: A boat, which moves at 18 miles per hour in water without a current, goes 80 miles upstream and 80 miles back again in 9 hours. Find the speed of the current to the nearest tenth.
I am very confused on how to work out this problem...
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
A boat, which moves at 18 miles per hour in water without a current, goes 80 miles upstream and 80 miles back again in 9 hours. Find the speed of the current to the nearest tenth.
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Let current speed be "c" .
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Upstream DATA:
distance = 80 miles ; rate = 18-c mph ; time = d/r = 80/(18-c) hrs.
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Downstream DATA:
distance = 80 miles ; rate = 19+c mph ; time = d/r = 80/(18+c) hrs
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Equation:
time up + time down = 9 hrs
80/(18-c) + 80/(18+c) = 9
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80(18+c) + 80(18-c) = 9(18-c)(18+c)
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2*80*18 = 9(324-c^2)
320 = 324-c^2
c^2 = 4
c = 2 mph (speed of the current)
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Cheers,
Stan H.

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