In the sequence x_{0}, x_{1}, x_{2}, ..., x_{n}, each term from x_{1} to x_{k} is 3 greater than the previous term, and each term from x_{k+1} to x_{n} is 3 less than the previous term, where n and k are positive integers and k < n. If x_{0} = x_{n} = 0 and if x_{k} = 15, what is the value of n?