Let U be a universal set with disjoint subsets A & B. n(U)=55, n(A)=25, n(B)=10. Find n(A' u B)

Answer 1

If subsets A and B are disjoint, there is no overlap (no intersection between them). Sometimes this is called being mutually exclusive. Think of it as a Venn Diagram. There are 25 people in circle A. There are 10 people in circle B. That means out of the 55 people within the diagram as a whole 55 - (25 + 10) = 20 people are still in the rectangle but outside of either circle.

When the question is asking for A' U B, they are asking for the people outside of Circle A or in Circle B...so in this case there are 30 total people outside of Circle A or in Circle B.

If you need more help with probability concepts, sets, and venn diagrams, please contact me for one-on-one sessions.