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Venn Diagram & Principle of Inclusion and Exclusion

Suppose there are N objects out of which there are exactly N_a objects of type a and N_b objects of type b. Also, suppose there are exactly N_{ab} objects that are of both type a AND b. Then, how many objects are NOT of type a OR bdouble-circles.jpg

The Venn diagram above illustrates this case.The number of objects outside the two oval regions equals N - (N_a + N_b - N_{ab}) = N - (N_a + N_b) + N_{ab}, and this is the answer.