  The charm content in $\eta'$ is estimated using the QCD U(1) axial anomaly
applied to $b\to s g g \to \eta' s$. We find $f_{\eta'}^{(c)} \approx - 3.1$
MeV for $m_c = 1.3$ GeV. Our estimate agrees with other independent methods by
Feldmann and Kroll, and Araki, Musakhanov and Toki. The resulting branching
ratios for $B \to \eta' K$ in the generalized factorization model are
marginally consistent with the CLEO data, although they are lower than the data
by $\sim 2$.
