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A boat leaves a town and travels to another town at an

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Senior Manager
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A boat leaves a town and travels to another town at an [#permalink] New post 14 Jun 2007, 08:15
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B
C
D
E

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A boat leaves a town and travels to another town at an average speed of 6 miles per hour. It returnes back at an average speed of 9 miles per hour. What is the average speed for the round-trip?

A) 7.1
B) 7.2
C) 7.3
D) 7.4
E) 7.5
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 [#permalink] New post 14 Jun 2007, 09:08
Because the first trip is only at 6 mph and the second trip is at 9 mph, the first trip takes 3/2 longer than the second trip...

so [6(3) + 9(2)]/5 = 7.2

think of it like a weighted average
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 [#permalink] New post 14 Jun 2007, 21:16
Yup it is B. Another quick way to do this would be to assume that the distance between the towns is 18 miles.

Then time1= 3 hours and time 2 = 2 hours

Total D = 36 / total t = 5 = 36/5 = 7.2
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 [#permalink] New post 14 Jun 2007, 21:47
Let the distance between the towns be x miles.

Time taken to the town = x/6 hours
Time taken to return = x/9

Average speed = 2x/(x+6 + x/9) = 7.2 miles/hr
  [#permalink] 14 Jun 2007, 21:47
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