gmatdriller wrote:In a specific river, the current is 6 miles per hour. A riverboat, which has a top speed of 18 miles per hour in still waters, makes daily round trips up the river. How far upstream (against the current) does the riverboat transport passengers if the boat travels at its top speed the entire time, and the round trip upstream and back takes 10 hours?
A: 54 miles B: 60 miles C: 72 miles D: 80 miles E: 90 miles
pls someone explain the question, possibly with diagrams...
Thanks. OAD
The rate upstream = boat rate - current rate = 18-6 = 12 miles per hour.
The rate downstream = boat rate + current rate = 18+6 = 24 miles per hour.
We can plug in the answers, which represent the distance in each direction.
Answer choice C: 72 miles
Time upstream + time downstream = 72/12 + 72/24 = 9 hours.
Since the total time must be just a bit longer -- 10 hours -- the distance must be just a bit greater.
The correct answer is
D.
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