Question 359496
let speed in still water = xmph
current speed = 3mph
upstream speed = x-3 mph
downstream speed = x+3 mph
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time = distance / speed
36/(x+3)+36/(x-3)=5
36(x-3)+36(x+3)/(x+3)(x-3)=5
36x-108+36x+108=5(x^2-9)
36x+36x=5x^2-45
5x^2-72x-45=0
5x^2-75x+3x-45=0
5x(x-15)+3(x-15)=0
(x-15)(5x+3)=0
x=15 OR x=-3/5 ignore 
x= 15 boat speed in still water
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