GMATGuruNY wrote:

DCS80 wrote:Can't follow this question for some reason. Maybe you guys can break it down and tailor it a bit better than the brief Quant explanation

131)

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

this question overwhelms me, and I understand sequences for the most part.


Thanks!!
Derek

xâ‚€ = 0 and x(k) = 15.
From xâ‚€ to x(k), each term is 3 greater than the previous term.
Thus:
xâ‚€ = 0.
x� = 3.
xâ‚‚ = 6.
x₃ = 9.
xâ‚„ = 12.
xâ‚… = 15.
Since x(k) = 15, k=5.

x(k) = 15 and x(n) = 0.
From x(k+1) -- in other words, from x₆ -- to x(n), each term is 3 less than the previous term.
Thus:
x₆ = 12.
x₇ = 9.
x₈ = 6.
x₉ = 3.
x�₀ = 0.
Since x(n) = 0, n=10.

The correct answer is D.