Question 172939
    The current in a stream moves at a speed of 3 mph. A boat travels 45 mi upstream and 45 mi downstream in a total time of 8 hr. What is the speed of the boat in still water?
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You will need to apply the "distance" formula:
d = rt
where
d is distance
r is rate or speed
t is time
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Solving for t:
d = rt
d/r = t
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Let x = speed of boat in still water
45/(x+3) + 45/(x-3) = 8
Multiplying both sides by:(x+3)(x-3)
45(x-3) + 45(x+3) = 8(x+3)(x-3)
45[(x-3) + (x+3)] = 8(x^2-9)
45[2x] = 8x^2-72
90x = 8x^2-72
0 = 8x^2-90x-72
0 = 4x^2-45x-36
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Since we can't factor, we solve with the quadratic equation.  Doing so will yield:
x = {12, -0.75}
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We can ignore the negative solution leaving us with
x = 12 mph (speed of boat in still waters)
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Details of quadratic follows:
*[invoke quadratic "x", 4, -45, -36 ]