  A major goal in B physics is measuring the CP-violating asymmetry in the
decay $B^0 \rightarrow \pi^+ \pi^-$. In order to determine one of the phase
angles in the CKM matrix from this decay it is necessary to determine the
influence of the penguin amplitude. Here we show how, using $SU(3)$ symmetry,
the penguin effect can be approximately determined from the ratio of the decay
rates of $B^0 \rightarrow K^+ \pi^-$ and $B^0 \rightarrow \pi^+ \pi^-$.
