SOLUTION: The speed of a stream is 6 mph. If a boat travels 90 miles downstream in the same time that it takes to travel 45 miles upstream, what is the speed of the boat in still water?

Question 954938: The speed of a stream is 6 mph. If a boat travels 90 miles downstream in the same time that it takes to travel 45 miles upstream, what is the speed of the boat in still water?
Answer by macston(5194) (Show Source): You can put this solution on YOUR website!
s=speed of boat in still water
90mi/(s+6mph)=45mi/(s-6mph) Multiply each side by (s+6)(s-6)
90mi(s-6mph)=45mi(s+6mph)
90s-540=45s+270 Add 540 to each side.
90s=45s+810 Subtract 45s from each side.
45s=810 Divide each side by 45.
s=18 ANSWER: The speed of the boat in still water is 18 mph
CHECK:
90mi/(18mph+6mph)=45 mi/(18mph-6mph)
90 mi/24mph=45mi/12mph
3.75 hours=3.75 mph
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