Question 475279
boat speed = x mph
current = 3 mph
against current speed = x-3
with current speed = x+3
let y be the distance .

downstream t= y/(x+3)............1
upstream 2t = y/(x-3)............2

multiply (1) by 2
2t=2y/(x+3)

Therefore y/(x-3)=2y/(x+3)
1/(x-3)=2/(x+3)
2(x-3)=x+3
2x-6=x+3
2x-x=6+3
x=9 mph speed of boat

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