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Sn = 2Sn-1 + 4 and Qn = 4Qn-1 + 8 for all n > 1. If S5 = Q4

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Author Message
TAGS:
Director
Director
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Status: Preparing for the 4th time -:(
Joined: 25 Jun 2011
Posts: 564
Location: United Kingdom
Concentration: International Business, Strategy
GMAT Date: 06-22-2012
GPA: 2.9
WE: Information Technology (Consulting)
Followers: 13

Kudos [?]: 423 [0], given: 217

GMAT Tests User
Sn = 2Sn-1 + 4 and Qn = 4Qn-1 + 8 for all n > 1. If S5 = Q4 [#permalink] New post 12 Jan 2012, 02:28
00:00
A
B
C
D
E

Difficulty:

(N/A)

Question Stats:

86% (02:20) correct 14% (03:06) wrong based on 13 sessions
Sn = 2Sn-1 + 4 and Qn = 4Qn-1 + 8 for all n > 1. If S5 = Q4 and S7 = 316, what is the first value of n for which Qn is an integer?

(A) 1
(B) 2
(C) 3
(D) 4
(E) 5

Any idea how to solve these guys? I have given up after scratching my head for a while.
_________________

Best Regards,
E.

MGMAT 1 --> 530
MGMAT 2--> 640
MGMAT 3 ---> 610 :-(

1 KUDOS received
Senior Manager
Senior Manager
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Joined: 30 Aug 2009
Posts: 290
Location: India
Concentration: General Management
Followers: 3

Kudos [?]: 95 [1] , given: 5

GMAT Tests User
Re: 1st value of n [#permalink] New post 12 Jan 2012, 03:00
1
This post received
KUDOS
enigma123 wrote:
Sn = 2Sn-1 + 4 and Qn = 4Qn-1 + 8 for all n > 1. If S5 = Q4 and S7 = 316, what is the first value of n for which Qn is an integer?
(A) 1
(B) 2
(C) 3
(D) 4
(E) 5

Any idea how to solve these guys? I have given up after scratching my head for a while.


Assuming I have understood the symbols used correctly answer is C- 3

S7 = 2S6 +4 = 316

S6 = 312/2 = 156 = 2S5 + 4

S5 = 152/2 = 76 = Q4

Q4 = 4Q3 + 8 = 76

Q3 = 68/4 = 17

Q3 = 4Q2 + 8 = 17

Q2 = 9/4 = 2.25

Q2 = 4Q1 + 8

Q1 will not be integer
Director
Director
avatar
Status: Preparing for the 4th time -:(
Joined: 25 Jun 2011
Posts: 564
Location: United Kingdom
Concentration: International Business, Strategy
GMAT Date: 06-22-2012
GPA: 2.9
WE: Information Technology (Consulting)
Followers: 13

Kudos [?]: 423 [0], given: 217

GMAT Tests User
Re: 1st value of n [#permalink] New post 12 Jan 2012, 21:11
Many thanks kp1811. Really appreciate your help.
_________________

Best Regards,
E.

MGMAT 1 --> 530
MGMAT 2--> 640
MGMAT 3 ---> 610 :-(

Re: 1st value of n   [#permalink] 12 Jan 2012, 21:11
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Sn = 2Sn-1 + 4 and Qn = 4Qn-1 + 8 for all n > 1. If S5 = Q4

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