Cardinality of $X \cup Y$? There are two sets $X$ and $Y$, where $|X| = 20$ and $|Y| = 32$. Which one of the following is a possible cardinality of $X \cup Y $? 
A. $5$ 
B. $10$ 
C. $24$ 
D. $40$
Correct answer was C, $|X \cup Y | = 24$, which i don't understand. Can anyone clear up this question? 

The correct answer must be D. Because we know that $A ,B \subseteq A \cup B$, therefore the cardinality of $A \cup B$ must be larger than or equal to $\max(20,32) =32$. The only choice compatible with this is 40.
