$x$	$18$	$19$	$20$	$21$	$22$
$F(x)$	$8$	$6$	$12$	$8$	$3$

Let $x$ be the ages of students in a class. Given the frequency distribution $F(x)$ above, determine the following probabilities:

(a) $P(x>19) =$

(b) $P(x=18) =$

(c) $P(x\le 21)=$