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Explanation:
Let's use an example to see why this is
A = {1,2,3,4,5,6,7}
B = {x,y,z}
A U B = {1,2,3,4,5,6,7, x,y,z}
We simply glue the two sets together to form one single big set. Because sets A and B have no overlap, this means we dont have to worry about tossing out duplicates (since there aren't any).
There are 7 items in set A, and 3 items in set B.
Therefore, we have 7+3 = 10 items in set A U B
The symbol U refers to the set union operator.
If you wanted to make a Venn Diagram, then you'd have two circles overlapping. In circle A will be the numbers 1 through 7. These values are not in the overlapped region. For circle B, the letters x,y,z are inside but not in the overlapped region. The overlapped region remains empty.
The set A U B talks about the collection of everything mentioned. It's the set of stuff in set A or in set B or in both sets.
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If you want a formula to write down on a notecard or reference sheet, then it would be
n(A U B) = n(A)+n(B)
where A and B are mutually exclusive, aka disjoint. This means they have nothing in the overlapped region
