Question

Suppose n(x)= 22 and n(y) is 13 what is the maximum number of elements X ∪Y can have ?

Answer 1

The inclusion/exclusion principle states that

$|X\cup Y|=|X|+|Y|-|X\cap Y|$

That is, the union has as many members as the sum of the number of members of the individual sets, minus the number of elements contained in both sets (to avoid double-counting).

Therefore, $|X\cup Y|$ will have the most elements when the sets $X$ and $Y$ are disjoint, i.e. $X\cap Y=\emptyset$, which would mean the most we can can in this case would be

$|X\cup Y|=|X|+|Y|=22+13=35$

(Note that $n(X)=|X|$ denotes the cardinality of the set $X$.)