Explanation:
800 in total.
555 health club members.
430 smokers.
320 health club members (that means out of the total of 555) were smokers.
that means that
555 - 320 = 235 health club members were not smokers.
and out of the 430 smokers 320 were also health club members.
that means that
430 - 320 = 110 smokers were not health club members.
the question how many were health club members or smokers means how many people were either health club members or smokers.
so, we need to combine the sets of health club members and smokers.
the number of elements in the resulting set is not just the sum of elements of both sets, because some elements are in both sets and would be counted twice.
therefore, we have a few alternatives :
1. add both sets and then subtract the common part (people being in both sets), as this removes the double counting :
555 + 430 - 320 = 665
2. start with the common part (people that are in both sets) and add the extra people in both sets (health club members that do not smoke, and smokers that are not health club members) :
320 + 235 + 110 = 665
3. start with one of the full sets (e.g. all health club members) and add the extra people of the other set (all smokers that are not also health club members) :
555 + 110 = 665
or all smokers plus all health club members that were not smokers :
430 + 235 = 665
as you can see, as expected, the answer is always the same :
out of the 800 nurses 665 were health club members or smokers.