= Solution
Model air as a dilute <ideal gas> of <molecules> described by the <hard sphere> model. The <number density> and <mean free path> are
$$
n=\frac{p}{k_BT},\qquad \lambda=\frac{1}{\sqrt2\,n\pi d^2}
=\frac{k_BT}{\sqrt2\,\pi d^2p}.
$$
The factor $\sqrt2$ accounts for the relative motion of the colliding <molecules>. A molecular <collision> diameter has to be estimated because none is supplied. Taking $d\simeq0.37\,\mathrm{nm}$, appropriate as an order-of-magnitude size for the main constituents of air, gives
$$
n=\frac{10^5}{(1.380649\times10^{-23})(298.15)}
\simeq2.43\times10^{25}\,\mathrm{m^{-3}},
\qquad
\lambda\simeq6.8\times10^{-8}\,\mathrm m.
$$
Thus \b[the <mean free path> is about $7\times10^{-8}\,\mathrm m$, or $70\,\mathrm{nm}$]. A slightly smaller assumed diameter $0.35\,\mathrm{nm}$ gives about $76\,\mathrm{nm}$, illustrating the intended precision of the estimate.
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