Answer
$P(correct~6-letter~words)=\frac{1}{240}\approx0.004167$
Work Step by Step
From (a): the sample space are the $720$ different 6-letter words. So, $N(S)=720$
$N(correct~6-letter~words)=3$
1) a single event
2) all the outcomes are equally likely
Use the Classical Method (page 259):
$P(correct~6-letter~words)=\frac{N(correct~6-letter~words)}{N(S)}=\frac{3}{720}=\frac{1}{240}\approx0.004167$
