Question 209211
    Julie and Eric row their boat (at a constant speed) 45 miles downstream for 5 hours, helped by the current. Rowing at the same rate, the trip back against the current takes 9 hours. Find the rate of the current.
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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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Let x = rate of the current
and y = rate Julie and Eric can row
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Since we have two variables, we'll need two equations:
Downstream:
5(y+x) = 45 (equation 1)
Upstream:
9(y-x) = 45 (equation 2)
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Solve equation 2 for y:
9(y-x) = 45
y-x = 5
y = 5+x
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Substitute the above into equation 1 and solve for x:
5(y+x) = 45
5(5+x+x) = 45
5+x+x = 9
2x = 4
x = 2 mph (rate of the current)