Question 889087: A man travels 40km down a river and back in 6 hours. If the speed of river current is 5km/hr, find speed of boat in still water. Answer by lwsshak3(11628) (Show Source):
You can put this solution on YOUR website! A man travels 40km down a river and back in 6 hours. If the speed of river current is 5km/hr, find speed of boat in still water.
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let x=speed of boat in still water
x+5=speed of boat in down river
x-5=speed of boat in up river
travel time=distance/speed
..
lcd:(x+5)(x-5)
40(x-5)+40(x+5)=6(x^2-25)
40x-200+40x+200=6x^2-150
6x^2-80x-150=0
3x^2-40x-75=0
(3x+5)(x-15)=0
x=-5/3 (reject)
or
x=15
speed of boat in still water=15 mph
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