Q1) A special sequence of numbers is written as 2,9,28,65,126...Find the ratio of the 7th term to the 15th term??
a)1/4
b)1/2
c)129/422
d)43/422
e)43/129
Answer D
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Sequence Problem
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soni_pallavi
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soni_pallavi wrote:Q1) A special sequence of numbers is written as 2,9,28,65,126...Find the ratio of the 7th term to the 15th term??
a)1/4
b)1/2
c)129/422
d)43/422
e)43/129
Answer D
I've never seen a GMAT question involving a sequence without any information regarding its pattern, and I'm pretty sure that I never will. The reason for this is that can be more than one way to describe the pattern within any sequence of numbers.
As such, I believe the question is out of scope.
Cheers,
Brent
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soni_pallavi
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Brent@GMATPrepNow
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On an official (GMATPrep) mock test?soni_pallavi wrote:Thanks Brent.
This question was on a mock test I took. There must have been some error.
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The formula for the sequence seems to be:soni_pallavi wrote:Q1) A special sequence of numbers is written as 2,9,28,65,126...Find the ratio of the 7th term to the 15th term??
a)1/4
b)1/2
c)129/422
d)43/422
e)43/129
Answer D
S(n) = n³ + 1.
To illustrate:
S(1) = 1³ + 1 = 2.
S(2) = 2³ + 1 = 9.
And so on.
Thus: S(7) = 7³ + 1 and S(15) = 15³ + 1.
A helpful identity:
a³ + b³ = (a+b)(a²-ab+b²)
Thus:
S(7) / S(15) =
= (7³ + 1³)/(15³ + 1³)
= (7+1)(7² - 7*1 + 1²) / (15+1)(15² - 15*1 + 1²)
= 8*43 / 16*211
= 43/422.
The correct answer is D.
I agree that this question is beyond the scope of the GMAT.
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I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
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