Numbering equations like 1a and 1b

use align end subequations instead of eqnarray or equation

\begin{subequations}
    \begin{align}
        p(w) &\sim \mathcal{N}(0, Q)\\
        p(v) &\sim \mathcal{N}(0, R)
    \end{align}
\end{subequations}

 See here.