Sequences 3 greater than, 3 less than

[chamb878]

I would really appreciate it if someone would be able to help me out on this one problem! :)

In the sequence X0, X1, X2, ,....ect Xn each term from X1 to Xk is 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 Xo=Xn=0 and if Xk=15, what is the value of n?
a) 5
b) 6
c) 9
d) 10
e) 15

This question is really confusing in general and can't seem to find how the two sequences relate to one another.

[Binit]

The most confusing part, for me, is k>n. If I ignore that I look at the series like this: 0,X1,X2,...,Xk-1,15,12,9,6,3,0.
This gives us another info: k=n+5. Since, k>0, n>=6. At this point we need some relation between terms X1, X2 etc. which is not given here(just Xk>Xk-1 is said).
So, n can be 6: 0,15,12,9,6,3,0.
n can be 9: 0,4,8,12,15,12,9,6,3,0. (I just chose 4,8 & 12 to show that n can be 9)
n can be 10: 0,3,6,9,12,15,12,9,6,3,0.
n can be 15 similarly.

[RonPurewal]

per the forum rules, we need a citation of the original source of this problem.

please provide one; thanks.