Solving equation with $n\log n$
No way to say. $T(n) = O(n \log n)$ means there exist constants $c &gt; 0, N_0$ so that for all $n \ge N_0$ it is $T(n) \le c n \log n$. They are anything, i.e., it might be valid only for ridiculously large $n$ (large $N_0$), or even for small $N_0$, like $N_0 = 10$, the constant $c$ could be such that all you know is that $T(100) \le \text{one milisecond}$ or $T(100) \le \text{one week}$.