  The first measurement that relates semileptonic and hadronic decay rates of
the $B_c^+$ meson is performed using proton-proton collision data corresponding
to 1.0 fb$^{-1}$ of integrated luminosity collected with the LHCb detector. The
measured value of the ratio of branching fractions, ${\cal B}(B_c^+ \to J/\psi
\pi^+)/{\cal B}(B_c^+\to J/\psi\mu^+\nu_\mu)=0.0469 \pm 0.0028 (stat.) \pm
0.0046 (syst.)$, is at the lower end of available theoretical predictions.
