The branching ratio ${\ensuremath{\mathrm{Br(b\rightarrow D \overline{D}X)}}}$ 
has been measured using hadronic Z decays
collected between 1993 and 1995 with the OPAL detector at LEP. The impact 
parameter significance of tracks in tagged b-jets is used to differentiate 
${\ensuremath{\mathrm{b\rightarrow D \overline{D}X}}}$ decays from other 
decays.  Using this result, the average number of charm and
anti-charm quarks produced in a bottom quark decay, $n_c$, is determined.