  The two body decays of $B_d$ and $B_s$ decays into $J/\psi M$, where $M$ is a
light meson, is studied under the very simple assumptions that the spectator
quark does not play a role in the decay of the weak heavy quark or antiquark.
This hypothesis leads to interesting relations between decay amplitudes. The
assumption of SU(3) symmetry leads to additional relations between the decay
amplitudes and in particular, the eight CP eigenstates $J/\psi K_S$, $J/\psi
\eta$, $J/\psi \eta^{\prime}$ and $J/\psi \pi^o$ are all given in terms of 0.
If agreement with experiment validates these assumptions the parameters over
determined by the results will give information about the ratio of penguin to
tree contributions to the "golden channel" $B^o \to J/\psi K_S$ decay and will
provide tests for the standard $\eta-\eta^{\prime}$ mixing, which assumes that
this mixing is determined by a single mixing angle, as well as determine the
value of the mixing angle. We also present tests of the standard
$\eta-\eta^{\prime}$ mixing involving semileptonic $D$ decays.
