\(27^n.9^n=9^{27}:81\) \(\Rightarrow\left(27.9\right)^n=9^{27}:9^2\) \(\Rightarrow\left(3^5\right)^n=9^{25}\) \(\Rightarrow\left(3^5\right)^n=\left(3^2\right)^{25}\) \(\Rightarrow3^{5n}=3^{50}\) \(\Rightarrow5n=50\) \(\Rightarrow n=10\) Vậy số tự nhiên n là 10