We know 445 employees take the train, and that 321 of these exclusively take the train. So 445 - 321 = 124 take both the train and bus.
Now, if
is the set of employees that take the bus and 
the set of employees that take the train, then
where
is the number of employees belonging to a general set 
.
So the probability that an employee takes either the bus or train is
Now, if
where
So the probability that an employee takes either the bus or train is
