We can start by finding angle $R$ using the law of cosines. The law of cosines states that $c^2 = a^2 + b^2 - 2ab\cos(C)$, where $a$, $b$, and $c$ are the sides of the triangle and $C$ is the angle opposite side $c$. In this case, $a=p=12$, $b=q=16$, and $c=r=20$.
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