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DS : Progression & Sequences

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DS : Progression & Sequences

by anujan007 » Thu Jul 05, 2012 4:24 pm
What is the 3rd term of Sequence A?

1) Each term is the equal to the sum of three times the previous term and 4.
2) The 5th term is 25% of the 6th term.

OA : C
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by anujan007 » Thu Jul 05, 2012 4:30 pm
I encountered this question on one of the sites with sample questions. I do not know the difficulty level of this question if at all it appears on the actual GMAT and also whether this is a standard question. Nevertheless it will surely not hurt to know how to approach such problems.

I started with the first statement here.

i. t3 = 3t2+4
t2 = 3t1+4
t1 = 3t0+4
None of the terms is known. So (i) is clearly insufficient.

Moving on to the second statement.

ii. t6 = 4t5

Insufficient

While solving both together I missed the trick. While enlisting it here, I realized my mistake. The correct approach would be as below.

Solving both together, we get

t6 = 4t5 and per the statement 1, t6 = 3t5 + 4

Hence t5=4. Using this we can get to t3.

I wish I could have originally been able to solve it there. This is the difference of solving in a timed situation and casually solving it. :(
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by Anurag@Gurome » Thu Jul 05, 2012 7:20 pm
anujan007 wrote:What is the 3rd term of Sequence A?

1) Each term is the equal to the sum of three times the previous term and 4.
2) The 5th term is 25% of the 6th term.
Statement 1: We can relate 3rd term to any other term in the sequence. But as we don't know the value of any of the terms of the sequence, we cannot determine 3rd term.

Not sufficient

Statement 2: We can only relate 5th term and 6th term in the sequence.

Not sufficient

1 & 2 Together: Now from statement 1, t(6) = 3*t(5) + 4 and from statement 2, t(6) = 4*t(4). Solving these two equations we can determine 5th term and hence 3rd term also.

Sufficient

The correct answer is C.
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