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Sequence

by greenwich » Fri Sep 24, 2010 2:32 pm
In the sequence X0, X1, X2, ..., Xn, each term from X1 to Xk is 3 greater than the previous term, and each term from Xk+1 to Xn is 3 less than the previous term, where n and k are positive integers and k<n. If X0=Xn=0 and if Xk=15, what is the value of n?

(A) 5
(B) 6
(C) 9
(D) 10
(E) 15
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by sanju09 » Sat Sep 25, 2010 12:32 am
greenwich wrote:In the sequence X0, X1, X2, ..., Xn, each term from X1 to Xk is 3 greater than the previous term, and *each term from Xk+1 to Xn is 3 less than the previous term, where n and k are positive integers and k<n. If X0=Xn=0 and if Xk=15, what is the value of n?

(A) 5
(B) 6
(C) 9
(D) 10
(E) 15

What a question

If xk = 15 and x0 = 0, then the series must begin as under

0, 3, 6, 9, 12, 15, ...

*In this continuity, x(k + 1) will be 12, and from here onwards, it would meet its end at 0 as under

... 12, 9, 6, 3, 0.

Hence, the series as a whole is a peculiar example of number symmetry

0, 3, 6, 9, 12, 15, 12, 9, 6, 3, 0

It has got 11 terms to it, hence n = [spoiler]10.


D[/spoiler]
The mind is everything. What you think you become. -Lord Buddha



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by sanju09 » Sat Mar 31, 2012 1:15 am
Received the following PM from pallasy to respond:
pallasy wrote:

I was reading your response to this post (https://www.beatthegmat.com/sequence-t67057.html) and was wondering why you said "It has got 11 terms to it, hence [spoiler]n = 10[/spoiler]." What's the logic you used to arrive at that conclusion?
Response:

As the sequence X0, X1, X2, ..., Xn has got its first term represented by x0, second term represented by x1, with xn representing its 11th term, hence 11 must be equal to [spoiler]n + 1, or n = 10[/spoiler].
The mind is everything. What you think you become. -Lord Buddha



Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com
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