Class a,
Class b sum
The sum of the number of class C elements A = the number of class elements+the number of class C elements+the number of class C elements-both class A and class B.
The number of elements of a class-the number of elements of both Class B and Class C-the number of elements of both Class A and Class C+the number of elements of both Class A, Class B and Class C, symbolically: ABBC ABBC ABBC =+-+.