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 arejustmarried but alsosomeof 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-wayintersectionsand 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...