Sorry that I abandon your notation... Anyhow, let
A: Those who are married
B: Those over 40
C: Those who passed physical examination
Now A does not just hold employees that are just married but also some of those over 40 and those who passed the physical exam. This must be the case since A+B+C > 1605...the total number of employees.
Now draw a venn-diagram of the situation. That is draw three circles A, B and C that all overlap each other slightly.
"gibberish here before..."
Now let's comute the union
By just adding
we will count each of the 2-way intersections and twice...so we have to subtract each of those once...
...but when we made this subtraction we also took away the 3-way intersection three times. As this intersection is counted three times in A+B+C we're now back to zero, hence we must add it once...
You have all the data for theese intersections and this might help you on the way...