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

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Math Expert
Joined: 02 Sep 2009
Posts: 57089
In a certain sequence, when you subtract the Mth element from the (M-1  [#permalink]

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02 Apr 2018, 02:17
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Difficulty:

45% (medium)

Question Stats:

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

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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.

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

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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.

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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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.

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Joined: 03 Jan 2018
Posts: 16
Location: Australia
WE: Information Technology (Computer Software)
Re: In a certain sequence, when you subtract the Mth element from the (M-1  [#permalink]

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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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Joined: 24 Oct 2016
Posts: 502
GMAT 1: 670 Q46 V36
GMAT 2: 690 Q47 V38
Re: In a certain sequence, when you subtract the Mth element from the (M-1  [#permalink]

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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

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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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