Concept: - The cardinality of a set A is the measure of the "number of elements" of the set. It is denoted by n(A). For example, a set containing 3 elements has a cardinality of 3 . - If A⊂B, then A∪B=B. Calculations: Since, A⊂B and B⊂C, therefore A∪B∪C=C. ⇒n(A∪B∪C)=n(C). ∴ The cardinality (number of elements) of A∪B∪C= cardinality (number of elements) of C.