= Solution
Away from thresholds and narrow <resonances>, photon-mediated annihilation gives one colour copy proportional to $Q_q^2$ relative to the <muon> cross-section. The accessible flavours are $u,d,s,c$, each with three colours, so
$$
\boxed{R\simeq3\sum_{q=u,d,s,c}Q_q^2
=3\left(\frac49+\frac19+\frac19+\frac49\right)=\frac{10}{3}\simeq3.3.}
$$
Bottom production is closed. QCD radiative corrections raise the leading parton-model result by a relative term beginning at $\alpha_s/\pi$; the requested estimate does not require a supplied value of $\alpha_s$.
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