Question 335571
let the speed in still water be x mph
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time taken = 20/60 = 1/3 hours
speed upstream = x-3
distance = speed * time
distance = (x-3)*1/3
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down stream speed = x+3
time = 10/60 = 1/6 hours
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distance = (x+3)*1/6
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1/6*(x+3)=1/3*(x-3)
3*(x+3)=6*(x-3)
3x+9 = 6x-18
-6x+3x = -9-18
-3x=-27
x= 9
x= 9 mph
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speed upstream = 9-3= 6 mph
time taken =1/3 hours 
Distance = 1/3 * 6 = 2 miles.