Let $X_{1}, X_{2}, \ldots, X_{n}$ be a random sample from $N\left(\mu, \sigma^{2}\right)$.
(a) If the constant $b$ is defined by the equation $\operatorname{Pr}(X \leq b)=0.90$, find the mle of $b$.
(b) If $c$ is given constant, find the mle of $\operatorname{Pr}(X \leq c)$.