\documentclass[12pt]{article}
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\begin{document}
\begin{eqnarray}
% Comments go here
\Pr(A|B) &=& \frac{\Pr(B|A)\,\Pr(A)}{\Pr(B)} \\
\Pr(A|B) &=& \mbox{"Posterior Probability of A"} \\
\Pr(A) &=& \mbox{"Prior Probability of A"} \\
\Pr(B) &=& \mbox{"Prior Probability of B" or "Normalizing Constant"}
\end{eqnarray}
\end{document}