SOLUTION: 4. 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

Question 80650: 4. 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
Answer by ankor@dixie-net.com(22740) (Show Source): You can put this solution on YOUR website!
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.
:
Let x = speed of the current:
Then:
(9-x) = speed upstream
(9+x) = speed downstream
:
Write a time equation: Time = Distance/speed
:
Time up + time down = 3 hrs
+ = 3
:
Multiply equation by: (9-x)(9+x) and you have:
12(9+x) + 12(9-x) = 3(9-x)(9+x)
:
108 + 12x + 108 - 12x = 3(81 - x^2)
:
216 = 243 - 3x^2
:
+3x^2 = 243 - 216
:
3x^2 = 27
:
Simplify, divide by 3
x^2 = 9
:
x = 3 mph speed of the stream:
:
:
Check; At 3 mph, speed up = 6 mph, speed down 12 mph:
12/6 + 12/12 = 3 hrs

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