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In each term of a sequence, 9 is added

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Magoosh GMAT Instructor
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In each term of a sequence, 9 is added [#permalink] New post 20 Mar 2013, 13:18
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A
B
C
D
E

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77% (01:56) correct 23% (01:22) wrong based on 115 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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Re: In each term of a sequence, 9 is added [#permalink] New post 20 Mar 2013, 13:35
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1rst term + 80 terms = 2 + 9+9+9+9+9+9+9+...+9 (80 times)

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

Answer D
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Veritas Prep GMAT Instructor
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Re: In each term of a sequence, 9 is added [#permalink] New post 20 Mar 2013, 21:10
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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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Re: In each term of a sequence, 9 is added [#permalink] New post 22 Mar 2013, 12:23
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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
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Re: In each term of a sequence, 9 is added [#permalink] New post 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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