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In a sequence of numbers in which each term after the first

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In a sequence of numbers in which each term after the first [#permalink] New post 14 Nov 2009, 06:38
00:00
A
B
C
D
E

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(N/A)

Question Stats:

100% (01:50) correct 0% (00:00) wrong based on 2 sessions
In a sequence of numbers in which each term after the first term is 1 more than twice the preceding term, what is the fifth term?

(1) The first term is 1.
(2) The sixth term minus the fifth term is 32.

Yeah friends, I just solved it completely to know whether the info is sufficient. It took me nearly 3 minutes. I am horrified. Can this be solved within 2 mins through any formula or expression ? Please-please-please tell me. I am dying to know.
Thanks in advance.
Tuck Thread Master
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Joined: 20 Aug 2009
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Schools: Stanford (in), Tuck (WL), Wharton (ding), Cornell (in)
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Re: O man I don't know the shortcut for this one [#permalink] New post 14 Nov 2009, 06:48
let the first term be x, then sequence looks like

1 - x
2 - 2x+1
3 - 4x+3
4 - 8x+7
5 - 16x+15
6 - 32x+ 31

(1) if x=1, then 5th term = 31, SUFFICIENT

(2) if (32x+31)-(16x+15) = 16x+16 = 32; x=1; 5th term = 31; SUFFICIENT

hence D

It shouldn't took more than 2 minutes IMO
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Re: O man I don't know the shortcut for this one [#permalink] New post 14 Nov 2009, 06:56
gmat620 wrote:
In a sequence of numbers in which each term after the first term is 1 more than twice the preceding term, what is the fifth term?

(1) The first term is 1.
(2) The sixth term minus the fifth term is 32.

Yeah friends, I just solved it completely to know whether the info is sufficient. It took me nearly 3 minutes. I am horrified. Can this be solved within 2 mins through any formula or expression ? Please-please-please tell me. I am dying to know.
Thanks in advance.



Question stem says - nth term =2* (n-1)th term +1
fifth term =?

1) first term = 1
second term = 2*first term+1
and so on.... you can find fifth term using the above formula but you don't really have to calculate.

SUFFICIENT

2) sixth term - fifth term = 32

sixth term = 2* (fifth term) +1

you can find fifth term here. SUFFICIENT

answer is 'D'
Tuck Thread Master
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Joined: 20 Aug 2009
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Location: Tbilisi, Georgia
Schools: Stanford (in), Tuck (WL), Wharton (ding), Cornell (in)
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Kudos [?]: 90 [0], given: 69

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Re: O man I don't know the shortcut for this one [#permalink] New post 14 Nov 2009, 07:26
yeah, it's important to remember that in DS questions you get no additional points for calculating something. You should only show wether it's possible to calculate (given this amount of info) or not.
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Re: O man I don't know the shortcut for this one [#permalink] New post 14 Nov 2009, 16:49
IMO D...
Statement 1 : Sufficient, because 1st term is given you can find the fifth term(no need to actually calculate it).
Statement 2 : Sufficient, let fifth term be X, therefore sixth term=2X+1=>2X+1-X=32=>x=31

Would take less than a minute..
Re: O man I don't know the shortcut for this one   [#permalink] 14 Nov 2009, 16:49
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