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There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the

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Math Revolution GMAT Instructor
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There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the [#permalink]

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08 Aug 2017, 18:10
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There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the sequence is the sum of the 3 previous numbers. r=?

A. -2
B. -1
C. 0
D. 1
E. 2
[Reveal] Spoiler: OA

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Re: There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the [#permalink]

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08 Aug 2017, 20:02
MathRevolution wrote:
There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the sequence is the sum of the 3 previous numbers. r=?

A. -2
B. -1
C. 0
D. 1
E. 2

This sequence is possible when r = -2, s = 5, t = -2 as r+s+t = 1.
To double check whether the numbers we have selected are correct,
we also have s + t + 1 = 4 to prove that the numbers chosen are correct.

Hence, it is clear that Option A(r = -2) is our correct answer.
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Re: There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the [#permalink]

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08 Aug 2017, 23:18
MathRevolution wrote:
There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the sequence is the sum of the 3 previous numbers. r=?

A. -2
B. -1
C. 0
D. 1
E. 2

We can solve this question by solving from last right number.

Given Each number of the sequence is the sum of the $$3$$ previous numbers.Therefore;

$$26 = (15 + 8 + 3) => 26$$

$$15 = (8 + 3 + 4) => 15$$

$$8 = (3 + 4 + 1) => 8$$

$$3 = (4 + 1 + s) => 5 + s$$

$$s = 3 - 5 => -2$$

$$4 = (1 + s + t)$$

$$4 = 1 - 2 + t => -1 + t$$

$$t = 4 + 1 => 5$$

$$1 = (s + t + r)$$

$$1 = -2 + 5 + r => 3 + r$$

$$r = 1 - 3 => -2$$

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Re: There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the [#permalink]

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10 Aug 2017, 01:14
==> You get r+t+s=1, from t+s+1=4, you get t+s=3, and from r+t+s=r+3=1, you get r=1-3=-2.

The answer is A.
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Re: There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the [#permalink]

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13 Aug 2017, 12:25
r,s,t,1,4,3,8,1,5,26
3= 4+1+t , t=-2
4=1+(-2)+s, s=5
1=r+5+(-2) , r=-2
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Re: There is a sequence of r, t, s, 1, 4, 3, 8, 15, 26. Each number of the   [#permalink] 13 Aug 2017, 12:25
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