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In a certain sequence, when you subtract the Mth element from the (M-1

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In a certain sequence, when you subtract the Mth element from the (M-1  [#permalink]

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New post 02 Apr 2018, 02:17
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A
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D
E

Difficulty:

  45% (medium)

Question Stats:

64% (01:55) correct 36% (01:58) wrong based on 112 sessions

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Re: In a certain sequence, when you subtract the Mth element from the (M-1  [#permalink]

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New post 02 Apr 2018, 02:29
Bunuel wrote:
In a certain sequence, when you subtract the Mth element from the (M-1)th element, you get twice the Mth element (M is any positive integer). What is the fourth element of this sequence?

(1) The first element of the sequence is 1.
(2) The third element of the sequence is 1/9.


Since the given rule tells us how to calculate one element in a sequence based on the one before/after it, then to know the value of the fourth element, all we need to know is the value of one of the elements.
We'll look for statements that give us this information, a Logical approach.

(1) gives us the first element and is sufficient.
(2) gives us the third element and is sufficient.

(D) is our answer.

Note that you don't actually need to calculate that the given rule is 'the Mth element is 1/3 the (M-1)th element' in order to solve the above.
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Re: In a certain sequence, when you subtract the Mth element from the (M-1  [#permalink]

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New post 02 Apr 2018, 02:54
Bunuel wrote:
In a certain sequence, when you subtract the Mth element from the (M-1)th element, you get twice the Mth element (M is any positive integer). What is the fourth element of this sequence?

(1) The first element of the sequence is 1.
(2) The third element of the sequence is 1/9.


Check NEWEST addition to Ultimate GMAT Quantitative Megathread:

'Sequences Made Easy - All in One Topic!'


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Re: In a certain sequence, when you subtract the Mth element from the (M-1  [#permalink]

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New post 24 Feb 2019, 14:42
Someone can explain me the question.. I think so there is a flaw in the question
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Re: In a certain sequence, when you subtract the Mth element from the (M-1  [#permalink]

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New post 08 Jun 2019, 12:54
Bunuel wrote:
In a certain sequence, when you subtract the Mth element from the (M-1)th element, you get twice the Mth element (M is any positive integer). What is the fourth element of this sequence?

(1) The first element of the sequence is 1.
(2) The third element of the sequence is 1/9.


Solution



b1 - b2 = 2b2
b1 = 3b2
b2/b1 = 1/3 => Common Ratio

Need: Any term in sequence

1) b1 = 1
b4 = 1 * (1/3)^3 = 1/27 => Sufficient


2) b3 = 1/3
b4 = (1/9) * (1/3) = 1/27 => Sufficient

ANSWER: D
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Re: In a certain sequence, when you subtract the Mth element from the (M-1   [#permalink] 08 Jun 2019, 12:54
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