A random sample of $n=81$ observations is drawn from a population with a mean equal to $24$ and a standard deviation equal to $18.$ Complete parts a through $e.$

a$.$ Find the probability that $\stackrel{}{x}$ is less than $21$

The probability that $\stackrel{}{x}$ is less than $21$ is

$($Type an integer or decimal rounded to four decimal places as needed.$)$