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# A swimmer makes a round trip up and down the river. It takes

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Manager
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A swimmer makes a round trip up and down the river. It takes [#permalink]  17 May 2008, 17:41
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A swimmer makes a round trip up and down the river. It takes him x hours. The next day he swims the same distance with the same speed in the still water. It takes him y hours. What can we say about x and y ?

a) x>y
b) x<y
c) x=y
d) x=y/2
e) none of the above
Director
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Re: PS: Swimmer [#permalink]  17 May 2008, 18:20
E

we don't know if the swimmer swamp up river during high tide, and down the river during high tide

or up river during high tide and down river during low tide

or up river during low tide and down river during low tide

or up river during low tide and down river during high tide
CEO
Joined: 17 Nov 2007
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Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
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Re: PS: Swimmer [#permalink]  17 May 2008, 18:30
1
KUDOS
Expert's post
A

1. fast way: Let imagine that the speed of the river is very close to the speed of the swimmer. So, when the swimmer is swiming up it can take a very long time and it is obvious that x>>y. So, A

2. usual way:

let V - the speed of the swimmer
U - the speed of the river
L - length of the round trip

$$y=\frac{L}{V}$$

$$x=\frac{\frac12*L}{V-U}+\frac{\frac12*L}{V+U}=L*\frac{V}{V^2-U^2}>L*\frac{V}{V^2-0^2}=L*\frac{1}{V}=y$$

So, x>y
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Re: PS: Swimmer [#permalink]  17 May 2008, 18:51
how can we tell anything about x and y's relation from the info given ? we dont know if the initial round trip was done in still water or not , etc.

thats why i went for E
CEO
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Re: PS: Swimmer [#permalink]  17 May 2008, 19:00
Expert's post
pmenon wrote:
how can we tell anything about x and y's relation from the info given ? we dont know if the initial round trip was done in still water or not , etc.

I think that river has always flow and additionally "up" as well as "down" in the phrase "A swimmer makes a round trip up and down the river." say that river has flow.
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Re: PS: Swimmer [#permalink]  17 May 2008, 19:02
i agree that if the question made any clear mention of the initial trip having the swimmer go against/with some current, the answer might be different ... but i dont know if we can make that assumption here.

Im stumped ... although I wont be too concerned if i was wrong because i dont think this is a real GMAT type question ... i hope
CEO
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Re: PS: Swimmer [#permalink]  17 May 2008, 19:14
Expert's post
Yes, all GMAT questions are airtight! I have not seen any vague questions from real GMAT sources.
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Manager
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Re: PS: Swimmer [#permalink]  18 May 2008, 06:31
Even if he swims the first day with current, he swims up and down, halfway with current against him, halfway with current behind him. Who's to say whether this is faster/slower/same as swimming with zero current?

Sure, we don't know about his conditioning relative to the level of effort required for this swim, but you could argue that the effort expended swimming against the current might not be fully offset by the effort saved swimming with it.

Either way, I'd go e.) because there's not enough detail about the first day's swim to model it discretely.
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Re: PS: Swimmer [#permalink]  12 Nov 2008, 01:56
seongbae wrote:
A swimmer makes a round trip up and down the river. It takes him x hours. The next day he swims the same distance with the same speed in the still water. It takes him y hours. What can we say about x and y ?

a) x>y
b) x<y
c) x=y
d) x=y/2
e) none of the above

I found this Q from old post.

I got A, don't know if I'm correct
Re: PS: Swimmer   [#permalink] 12 Nov 2008, 01:56
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