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# gmat prep ??

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Director
Joined: 23 May 2008
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gmat prep ?? [#permalink]

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16 Sep 2008, 00:20
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Intern
Joined: 06 Sep 2008
Posts: 4
Re: gmat prep ?? [#permalink]

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16 Sep 2008, 02:40
Use the AP formulae

Nth term = a + (n-1)*d

where d is the difference between n+1 and nth term and a is the 1st term. Should give you n = 10
Intern
Joined: 25 Mar 2006
Posts: 9
Location: Kolkata, India
Re: gmat prep ?? [#permalink]

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16 Sep 2008, 04:04
tn = t(n-1) - 3 => tn - t(n-1) = -3 => common diff. of AP = d = -3

Also, tn = t1 + (n-1).d = 23 + (n-1)(-3) =-4
=> n = 10.
SVP
Joined: 21 Jul 2006
Posts: 1508
Re: gmat prep ?? [#permalink]

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16 Sep 2008, 06:34
It is actually simpler than how it is presented. First of all, tn = t(n-1) - 3 basically means t2 = t1 - 3. So, t3 = t2 - 3. so basically, every bigger t will be its previous number minus 3. So:

t1 = 23
t2 = 20----> (23-3)
t3 = 17----> (20-3)
...
...
...
...
t10 = -4

So it's 10.

Director
Joined: 23 May 2008
Posts: 799
Re: gmat prep ?? [#permalink]

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16 Sep 2008, 08:34
thanks....nice explanation tarek!
SVP
Joined: 21 Jul 2006
Posts: 1508
Re: gmat prep ?? [#permalink]

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16 Sep 2008, 14:16
SVP
Joined: 17 Jun 2008
Posts: 1529
Re: gmat prep ?? [#permalink]

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18 Sep 2008, 00:47
I approached as follows.

We have first term as 23 and in order to get the value of term as -4, we need to get the total difference as -4-23 = -27.

Here, difference between two consecutive terms is -3. Hence, 9 terms will give a total difference of -27 and first term + 9 more terms will be 10th term.

Hence, 10 should be the answer.
SVP
Joined: 21 Jul 2006
Posts: 1508
Re: gmat prep ?? [#permalink]

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18 Sep 2008, 04:20
scthakur wrote:
I approached as follows.

We have first term as 23 and in order to get the value of term as -4, we need to get the total difference as -4-23 = -27.

Here, difference between two consecutive terms is -3. Hence, 9 terms will give a total difference of -27 and first term + 9 more terms will be 10th term.

Hence, 10 should be the answer.

interesting approach!
Re: gmat prep ??   [#permalink] 18 Sep 2008, 04:20
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