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# Gmatprep - sequence

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Intern
Joined: 04 Feb 2007
Posts: 29

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08 Feb 2007, 20:54
00:00

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Question Stats:

50% (00:00) correct 50% (02:04) wrong based on 2 sessions

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OA : [color=violet]C
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Senior Manager
Joined: 04 Jan 2006
Posts: 276

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08 Feb 2007, 21:03
Question: Given x1, x2, x3, ...; What is x1?

...(1) Xi = Xi-1/2
e.g. X2 = X1/2, X3 = X2/2
No number to find x1. INSUFF

...(2) x5 = x4/(x4 + 1)
No number to find x1. INSUFF

...(1) + (2);

Now we know the format Xi = Xi-1/2
and X5 = X4/(X4+1)

Clearly, X4 + 1 = 2
X4 = 2-1 = 1

X4 = (X3)/2 = 1; X3 = 2
X3 = (X2)/2 = 2; X2 = 4
..Keep going and you will know X1. Thus, (1) + (2) SUFF.
Manager
Joined: 10 Dec 2005
Posts: 112

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08 Feb 2007, 21:49
1
KUDOS
joyce wrote:
OA : [color=violet]C

From condition 1 we know x2 =x1/2 ==> x1 = 2x2 is not sufficient to give us the value of x1

From condition 2 we know x5 = x4/x4+1 hard to dervie what is the value of x1 from this hence not sufficient

condition 1 and 2 together

x5 = x4/2 = x4/x4 +1 ==> x4+1 = 2 ==> x4 = 1------ I

we also know from condition 1 that x4 = x3/2 = 1 (from I)

hence x3 = 2----II

again from condition 1 we know x3 = x2/2 = 2 (from II)
x2 = 4

we know x1 = 2x2 = 2*4 = 8

_________________

"Live as if you were to die tomorrow. Learn as if you were to live forever." - Mahatma Gandhi

Intern
Joined: 09 Sep 2007
Posts: 10

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17 Jun 2008, 10:40
Hi guys,
i was going thru this post and came across one question.

statement 1+2 gives us
x4/2 = x4/(x4+1)
here we are equating x4+1=2 => x4=1

why dont we cross multiply here which gives us x4=0 or 1?
Intern
Joined: 22 Jun 2009
Posts: 2

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29 Jun 2009, 13:52
There are two possible values based on how you solved for X4. However, the question stem says the progression consists only of positive numbers. 0 is neither positive nor negative (it is even though). Hence you can rule out X4=0 as in invalid solution and proceed with X4.
Re: Gmatprep - sequence   [#permalink] 29 Jun 2009, 13:52
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