SOLUTION: Rita rows 12km upstream and 12 km downstream in 3 hours. The speed of her boat in still water is 9km/hour. Find the speed of the stream.

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Question 139347: Rita rows 12km upstream and 12 km downstream in 3 hours. The speed of her boat in still water is 9km/hour. Find the speed of the stream.
Answer by checkley77(12844) About Me  (Show Source):
You can put this solution on YOUR website!
12/(9-X)+12/(9+X)=3
(12[9+X]+12[9-X])/(9-X)(9+X)=3
(108+12X+108-9X)/(81-X^2)=3 NOW CROSS MULTIPLY.
3(81-X^2)=108+12X+108-9X
243-3X^2=216+3X
-3X^2-3X+243-216=0
-3X^2-3X+27=0
-3(X^2+X-9)=0
USING THE QUADRATIC EQUATION WE GET:x+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
X=(-1+-SQRT[1^1-4*1*-9])/2*1
X=(-1+-SQRT[1+36)/2
X=(-1+-SQRT37)/2
X=(-1+-6.083)/2
X=(-1+6.083)/2
X=5.083/2
X=2.54 ANSWER FOR THE SPEED OF THE CURRENT.
PROOF:
12/(9-2.54)+12/(9+2.54)=3
12/6.45+12/11.54=3
1.86+1.04=3
2.9~3