Sets and reasoning
Adding two set totals counts the overlap twice, so subtract the intersection once to get the union.
A worked example
If n(A) = 18, n(B) = 14 and n(A ∩ B) = 6, then n(A ∪ B) = 18 + 14 - 6 = 26.
The mistake people actually make
'Neither' means taking the union away from the whole group, not away from one set. With three sets, subtract each pair and then add the triple overlap back on.