21. Suppose that a function $M(n)$, where $n$ is a positive integer, is defined by
$$
M(n)=\left\{\begin{array}{ll}
n-10 & \text { if } n>100 \\
M(M(n+11)) & \text { if } n \leq 100
\end{array}\right.
$$

How many solutions does the equation $M(n)=91$ have?