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September 18, 2014

September 18, 2014

Posted by **zshen** on Tuesday, January 15, 2013 at 5:48am.

- math -
**Reiny**, Tuesday, January 15, 2013 at 9:47amLet the speed of the boat in still water be x km/h

time upstream = 8/(x-3)

time downstream = 8/(x+3)

8/(x-3) + 8/(x+3) = 2

times (x+3)(x-3)

8(x+3) + 8(x-3) = 2(x+3)(x-3)

8x + 24 + 8x - 24 = 2x^2 - 18

2x^2 -16x - 18 = 0

x^2 - 8x - 9 = 0

(x-9)(x+1) = 0

x = 9 or x = -1, a negative x is not likely

x = 9

check:

speed upstream = 6 km/h, time = 8/6 or 4/3 hrs

speed downstrem = 12 km/h, time = 8/12 = 2/3 hr

total time = 4/3 + 1/3 = 2

all is good!

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