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Question

The speed of a river current is 3 mph. If a boat travels 20 miles downstream in the same time that it takes to travel 10 miles upstream, find the speed of the boat in still water.

Answer 1

distance=rate*time

d=rt

20=(r+3)t (r=speed of the boat)

10=(r-3)t  (r=speed of the boat)

20/(r+3)=t

10/(r-3)=t

set them equal to each other

20/(r+3)=10/(r-3)

cross multiply

20(r-3)=10(r+3)

20r-60=10r+30

20r-10r=60+30

10r=90

r=90/10

r=9 mph is the speed of the boat in still water