The inclusion-exclusion principle counts elements in the union of sets by correcting for overcounting in intersections. For two sets: |A U B| = |A| + |B| - |A ∩ B|. For three sets: |A U B U C| = |A| + |B| + |C| - |A ∩ B| - |B ∩ C| - |A ∩ C| + |A ∩ B ∩ C|. The pattern generalizes: add individual cardinalities, subtract pairwise intersections, add triple intersections, subtract quadruple intersections, and so on with alternating signs. In JEE, this is typically tested through word problems: "In a class of N students, how many study at least one subject?" given data about individual and pairwise enrollments. The complementary count — elements in none of the sets = |U| - |A U B U ...| — is also frequently asked.