Question 761851
Tony'a outboard can drive her boat at 7mph in still water. It takes her 10 minutes more to reach her friends camp 4 miles up the river than to return to her camp down river. What is the speed of the current? 
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The speed is in mi/hr, so change 10 mins to 1/6 hour
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c = current
d = r*t
4 = (7-c)*(t + 1/6) = (7+c)*t
7t - ct -c/6 + 7/6 = 7t + ct
That won't work
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t = 4/(7+c)
t + 1/6 = 4/(7-c)
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Sub for t
4/(7+c) + 1/6 = 4/(7-c)
(24 + 7 + c)/(6(7+c)) = 4/(7-c)
(31 + c)*(7-c) = 24(7+c)
-c^2 + -24c + 217 = 24c + 168
c^2 + 48c - 49 = 0
(c-1)*(c+49) = 0
c = 1 mi/hr