13. Let $x[n]$ denote $x^{x^{\prime}}$, where there are $n$ terms of $x$. What is the minimum value of $n$ such that $9[9]<3[n]$ ?
(For example, $3[2]=3^{3}=27 ; 2[3]=2^{2^{2}}=16$.)
13. Answer. 10
Solution. Suppose $9[m]3[9]$, hence the minimum value of $n$ is 10 .