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# In each term of a sequence, 9 is added

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Magoosh GMAT Instructor
Joined: 28 Dec 2011
Posts: 2025
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Kudos [?]: 1984 [0], given: 29

In each term of a sequence, 9 is added [#permalink]  20 Mar 2013, 13:18
Expert's post
00:00

Difficulty:

25% (low)

Question Stats:

77% (01:56) correct 23% (01:22) wrong based on 111 sessions
In each term of a sequence, 9 is added to get the next term. If the first term is 2, what is the eighty-first term?
(A) 632
(B) 695
(C) 713
(D) 722
(E) 731

For more on sequences, as well as a complete explanation of this question, see:
http://magoosh.com/gmat/2012/sequences-on-the-gmat/

Mike
[Reveal] Spoiler: OA

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Mike McGarry
Magoosh Test Prep

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Joined: 24 Jan 2013
Posts: 81
Followers: 5

Kudos [?]: 61 [3] , given: 6

Re: In each term of a sequence, 9 is added [#permalink]  20 Mar 2013, 13:35
3
KUDOS
1rst term + 80 terms = 2 + 9+9+9+9+9+9+9+...+9 (80 times)

2 + (9 x 80) = 2 + 720 = 722

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Veritas Prep GMAT Instructor
Joined: 11 Dec 2012
Posts: 313
Followers: 59

Kudos [?]: 184 [0], given: 66

Re: In each term of a sequence, 9 is added [#permalink]  20 Mar 2013, 21:10
Expert's post
mikemcgarry wrote:
In each term of a sequence, 9 is added to get the next term. If the first term is 2, what is the eighty-first term?
(A) 632
(B) 695
(C) 713
(D) 722
(E) 731

For more on sequences, as well as a complete explanation of this question, see:
http://magoosh.com/gmat/2012/sequences-on-the-gmat/

Mike

Quick tip on this type of question: the GMAT is frequently counting on you (pun intended) to miscount and think the answer is off by one iteration in either direction, so the correct answer tends to not be the biggest or smallest you find. This is far from ironclad, but if you find yourself picking the biggest number (in this case E), you've probably fallen for a classic GMAT trap.
The solution outlined by johnwesley above is excellent, but a lot of students find themselves doing 81*9+2 = 731 and falling into the trap.

Hope this helps!
-Ron
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Joined: 25 May 2012
Posts: 7
Location: United States
Concentration: Finance
WE: Corporate Finance (Non-Profit and Government)
Followers: 0

Kudos [?]: 7 [0], given: 0

Re: In each term of a sequence, 9 is added [#permalink]  22 Mar 2013, 12:23
1
This post was
BOOKMARKED
Xo = 2
X1 = Xo + 9 = 2 + 9
X2 = X1 + 9 = (2 + 9) + 9
X3 = X2 + 9 = ((2+ 9) +9) +9

Xn = 2 + 9n
X80 = 2 + 9(80)

X80 = 722
D
Intern
Joined: 17 May 2013
Posts: 49
GMAT Date: 10-23-2013
Followers: 0

Kudos [?]: 3 [0], given: 8

Re: In each term of a sequence, 9 is added [#permalink]  26 Jul 2013, 05:52
There is formula where you can find the nth term.

a + (n-1)d

a = first term = 2
d = common difference = 9

so for 81th term

2 + (81-1)9 = 722
Re: In each term of a sequence, 9 is added   [#permalink] 26 Jul 2013, 05:52
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