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GmatKiss: Legendary Member; Posts: 2789; Joined: Tue Jul 26, 2011 12:19 amFollowed by: 43 members; Location: Chennai, India; Thanked: 206 times; GMAT Score:640

by GmatKiss » Thu May 17, 2012 11:53 am

In a sequence of terms in which each term is three times the previous term, what is the fourth term?

(1) The first term is 3.

(2) The second-to-last term is 3^10.

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Source: Beat The GMAT — Data Sufficiency | Thu May 17, 2012 11:53 am

neelgandham: Community Manager; Posts: 1060; Joined: Fri May 13, 2011 6:46 amFollowed by: 52 members; Location: Utrecht, The Netherlands; Thanked: 318 times

by neelgandham » Thu May 17, 2012 12:41 pm

In a sequence of terms in which each term is three times the previous term, what is the fourth term? The series is of the form x, 3x, 3^2 * x ........3^(n-1) * x. The question can be rephrased to.
If x, 3x, 3^2 * x ........3^(n-1) * x is a series, what is the value of 27*x

(1) The first term is 3.

First term in the series = x = 3. Then 27x = 27*3 = 81> Statement 1 is sufficient to answer the question.

(2) The second-to-last term is 3^10.

3^(n-2) * x = 3^10. There are 2 variable n and x but just one equation. So, Statement 2 is Insufficient to answer the question.

IMO A

Anil Gandham
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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Thu May 17, 2012 6:37 pm

GmatKiss wrote:In a sequence of terms in which each term is three times the previous term, what is the fourth term?

(1) The first term is 3.

(2) The second-to-last term is 3^10.

(1) If the first term is 3, second term is 3^2 = 9, third term is 3^3 = 27, and fourth term is 3^4 = 81.
So, statement (1) alone is SUFFICIENT to answer the question.

(2) Knowing the second-to-last term will not help since we need to know the number of terms in the sequence as well.
So, statement (2) alone is NOT sufficient.

The correct answer is A.

Anurag Mairal, Ph.D., MBA
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