If n(a)= 20, n(b)=13, and n(AnB)=8, find n(A?B)

Question

If n(a)= 20, n(b)=13, and n(A∩B)=8, find n(A∪B)

Answer 1

All sets are denoted using Upper Case Letters.

To answer your question, 

n(AUB)=n(A)+n(B)-n(AnB)

   n(AUB)  = 20+13-8 = 25.

Answer 2

Since you mentioned Venn Diagrams, image the following drawing of two interconnecting circles:

 

          A                                                    B

 

 

                12                8                  5

 

 

 

 

 

 

Find the number in the union of A and B (that is, count 12+8+5 = 25).