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$\renewcommand{\d}{\displaystyle}\newcommand{\N}{\mathbb N}\newcommand{\Z}{\mathbb Z}\newcommand{\st}{:}$

Find the following cardinalities:

$|A \cap B|$ when $A = \{x \in \N \st x \le 20\}$ and $B = \{x \in \N \st x \mbox{ is prime} \}\text{.}$