Venn diagrams are the diagrams that are used to represent the sets, relation between the sets and operation performed on them, in a pictorial way. Venn diagram, introduced by John Venn (1834-1883), uses circles (overlapping, intersecting and non-intersecting), to denote the relationship between sets. A Venn diagram is also called a set diagram or a logic diagram showing different set operations .
For example, a set of natural numbers is a subset of whole numbers, which is a subset of integers. The relation between the sets of natural numbers, whole numbers and integers can be shown by the Venn diagram, where the set of integers is the universal set:

We can use the below formula to solve the problems based on two sets.
n(X ∪ Y) = n(X) + n(Y) – n(X ∩ Y)

The formula used to solve the problems on Venn diagrams with three sets is given below:
n(A ∪ B ∪ C) = n(A) + n(B) + n(C) – n(A ∩ B) – n(B  ∩ C) – n(A  ∩ C) + n(A ∩ B ∩ C)