SOLUTION: the current of a river is 3 miles/hour. it takes a motorboat a total of 3 hours to travel 12 miles upstream and return 12 miles downstream. what is the speed of the boat in still w

Question 946745: the current of a river is 3 miles/hour. it takes a motorboat a total of 3 hours to travel 12 miles upstream and return 12 miles downstream. what is the speed of the boat in still water
Answer by lwsshak3(11628) (Show Source): You can put this solution on YOUR website!
the current of a river is 3 miles/hour. it takes a motorboat a total of 3 hours to travel 12 miles upstream and return 12 miles downstream. what is the speed of the boat in still water
***
let x=speed of boat in still water
x+3=speed of boat downstream
x-3=speed of boat upstream
travel time=distance/speed
..
12/(x+3)+12/(x-3)=3
lcd:(x+3)(x-3)
12(x-3)+12(x+3)=3(x+3)(x-3)
12x-36+12x+36=3(x^2-9)=3x^2-27
3x^2-24x-27=0
x^2-8x-9=0
(x+1)(x-9)=0
x=9
speed of boat in still water=9 mph
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