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How many numbers of this sequence of 342 numbers are odd?

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How many numbers of this sequence of 342 numbers are odd?

by vinni.k » Mon Sep 09, 2013 9:28 am
How many numbers of this sequence of 342 numbers are odd?
(1) The sequence starts with -1,3,-3,..........
(2) Except for the 1st two numbers, every number in the sequence is the product of the two immediately preceding numbers

OA is C

[spoiler]Should a pattern be considered after reading the first 3 terms -1,3,-3,..........
I mean to say, i considered a pattern, a bit early i.e between -1 & 3 there is a gap of 4
between 3 & -3 there is a gap of 6
then between -3 & 1 there is a gap of 4 and then again 6 and so on.

-1,3,-3,1,-5.......

thus this gives odd numbers.
I am considering patterns early. Should there be more terms to confirm this pattern ? or is it a wrong pattern as nothing is defined.[/spoiler]

Looking forward for an explanation.

Thanks & Regards
Vinni
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Source: — Data Sufficiency |
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by Java_85 » Mon Sep 09, 2013 10:49 am
Hi vinni.k,

(1) is not enough to answer the question.
(2) alone is also not sufficient. == > C or E

(1)(2) -1 3 -3 -9 27 ...

in this sequence all numbers will be odd, because all are multiples of 3 and only will have factor 3. ==> C

You don't need to calculate the gaps, just see how the series is going and weather an even factor have a way to get into the series or not, here it does not.

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by GMATGuruNY » Mon Sep 09, 2013 12:54 pm
vinni.k wrote:How many numbers of this sequence of 342 numbers are odd?
(1) The sequence starts with -1,3,-3,..........
(2) Except for the 1st two numbers, every number in the sequence is the product of the two immediately preceding numbers

OA is C
Statement 1: The sequence starts with -1,3,-3,....
Given only the first 3 terms, we cannot determine how each value in the sequence is calculated.
Thus, there is no way to know how many of the 342 terms will be odd.
INSUFFICIENT.

Statement 2: Except for the 1st two numbers, every number in the sequence is the product of the two immediately preceding numbers.
If the first two terms are 1 and 3, then all of the terms in the sequence will be ODD.
If the first two terms are 2 and 4, then all of the terms in the sequence will be EVEN.
INSUFFICIENT.

Statements combined:
Since we know the first two terms and the formula for computing every subsequent term, we can calculate every term in the sequence.
Thus, we can determine how many of the 342 terms will be odd.
SUFFICIENT.

The correct answer is C.
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by vinni.k » Tue Sep 10, 2013 9:29 am
Thank you :)

Vinni
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