= Solution
Let $z_c$ be the number of <conduction electrons> per atom. Then $n=z_c n_a$ and the Drude relations give $\sigma=ne\mu=ne^2\tau/m_e$. Consequently
$$
\boxed{\mu=\frac{5.24\times10^{-3}}{z_c}\ \mathrm{m^2\,V^{-1}\,s^{-1}},\qquad
\tau=\frac{2.98\times10^{-14}}{z_c}\ \mathrm s}.
$$
Under an explicit one-electron-per-atom assumption, \b[$\mu=5.24\times10^{-3}\ \mathrm{m^2\,V^{-1}\,s^{-1}}$ and $\tau=2.98\times10^{-14}\ \mathrm s$]. The carrier count is not specified: monatomic does not by itself imply monovalent, so the boxed dependence on $z_c$ is the general result supported by the data. The collision time here is the transport <relaxation time> inferred within the free-electron model, not a measurement of every microscopic scattering event.
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