Question 170728
 Rita rows 12 km upstream and 12 km downstream in 3 hours. The speed of her boat in still water is 9 km/hr. Find the speed of the stream. 
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Call the speed of the stream w (for water).
Going upstream, her speed (with respect to the land) is 9-w
Going downstream, it's 9+w
See that?
The time it takes is the distance (s) over the speed.
So upstream it's 12/(9-w) hours
Downstream is 12/(9+w) hours
Make sense so far?
Now, 12/(9-w) + 12/(9+w) = 3
It's no longer a "word problem", it's one equation in one unknown.
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12/(9-w) + 12/(9+w) = 3
4/(9-w) + 4/(9+w) = 1
Multiply thru by (9-w)*(9+w)
4(9+w) + 4(9-w) = w^2 - 81
72 = w^2 - 81
w^2 = 9
w = 3 km/hr (also -3, but that makes no sense and can be discarded)