SOB 226 [Typical]
Suppose $X$, $Y$ and $Z$ are sets with
$$|X| = 26
\quad |Y| = 21 \quad |Z| = 32,$$
$$|X \cap Y| = 19, \quad |X\cap Z| = 19,
\quad |Y \cap Z| = 13,\quad |X \cap Y \cap Z| = 9.$$
Use the inclusion-exclusion principle to calculate $|X \cup Y \cup Z|.$
$|X \cup Y \cup Z| = 37$