Posted - 12/01/2013 : 00:00:01 GMAT-the official guide, #131 In the sequence X0, X1,X2,X3....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=X0=0and if Xk =15 what is the value of N" Ok I can sort of visualize this thing, something starts at 0 and increases up to 15 and then decreases back down to 0. And I follow the simple explanation of k=5. I just can't see "then Xn=Xk-(n-k)(3)". Maybe I am just brain tired. The sequence is from k down to n. I am trying to put myself in the mind that constructed this relationship expressed , and I just don't see the "why's" of how this was assembled. Can you? and explain it to me?

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Joe Komagawa

Posted - 12/01/2013 : 00:27:05 sorry, a) 5, B) 6, C) 9, D) 10, E) 15, answer is D