Solution
Begin by defining events.
S= the event the employee leaves because of salary.
W= the event the employee leaves because of work assignments.
P(S)=.30 P(W)=.20 P(S?W)=.12
The first two are marginal probabilities, the last is the joint probability.
P(S?W) = P(S) + P(W) - P(S?W)