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# A man can row at a speed of 5 km/hr in still water. If he

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Manager
Joined: 28 Jun 2004
Posts: 90

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A man can row at a speed of 5 km/hr in still water. If he [#permalink]

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18 Aug 2004, 15:38
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

A man can row at a speed of 5 km/hr in still water. If he rows a certain distance upstream and back to the starting point in a river which flows at 1.5 km/hr, what is his average speed for the double journey?

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GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5036

Kudos [?]: 437 [0], given: 0

Location: Singapore

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18 Aug 2004, 19:15
average speed = total distance/total time

let distance of river be d
so assuming the journey down the river is in the direction of the river flow, then
time= d/ 6.5

and the journey up the river opposing the direction of the river is
time = d/3.5

total time = 40d/91 km/h

average speed = 2d/(40d/91)=4.55 km/h

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Director
Joined: 20 Jul 2004
Posts: 590

Kudos [?]: 156 [0], given: 0

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19 Aug 2004, 22:48
there is also a formula for these.
Let u be the upstream speed and v be the downstream speed.
Ave speed = 2uv/(u+v) = 2 x 6.5 x 3.5 . (6.5 + 3.5) = 4.55

Kudos [?]: 156 [0], given: 0

GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5036

Kudos [?]: 437 [0], given: 0

Location: Singapore

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20 Aug 2004, 00:30
hi hardworker, it's a neat formula, but it's better to work it out without formulas. The working isn't too long either. Much bette to save the brains remembering idioms for the verbal section !

Kudos [?]: 437 [0], given: 0

20 Aug 2004, 00:30
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