Inclusion-Exclusion Formula

The Inclusion-Exclusion Formula helps calculate the total number of items in overlapping groups without double-counting. Imagine counting people in two clubs: some belong to both. To find the total unique members, you add the members from each club separately and then subtract the people counted twice (those in both clubs). For more groups, this idea extends by adding the sizes of individual groups, subtracting those in overlapping pairs, adding back those in triple overlaps, and so on. It ensures an accurate count by balancing the overlaps correctly.