SOLUTION: a boat traveled 9 miles downstream and returned. The total trip took 2.25 hours. If the speed of the current was 3mph, find the speed of the boat in still water.

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Question 373944: a boat traveled 9 miles downstream and returned. The total trip took 2.25 hours. If the speed of the current was 3mph, find the speed of the boat in still water.
Answer by mananth(16946) (Show Source):
You can put this solution on YOUR website!
a boat traveled 9 miles downstream and returned. The total trip took 2.25 hours. If the speed of the current was 3mph, find the speed of the boat in still water.
speed of boat in still water = xmph
speed of current = 3 mph
..
upstream speed = x-3
downstream speed = x+3
...
distance traveled = 9 miles
..
time upstream + time downstream = total time
t=d/r
9/(x+3) +9/(x-3)=2.25
LCD = (x+3)(x-3)
9(x-3)+9(x+3)/(x^2-9)=9/4......(2 1/4 = 9/4)
9x-27+9x+27=9(x^2-9)/4
18x=(9x^2-81)/4
72x=9x^2-81
9x^2-72x-81=0
/9
x^2-8x-9=0
x^2-9x+x-9=0
x(x-9)+1(x-9)=0
(x-9)(x+1)=0
x=9 ignore x=-1
speed of boat = 9 mph
...
m.ananth@hotmail.ca