A $\text{CSTR}$ with a volume of $100 \mathrm{~m}^{3}$ is operated in cell recycle mode. At a volumetric flow rate of $10 \mathrm{~m}^{3}$ day ${ }^{-1}$ and effluent biomass of $20 ~\mathrm{mg} \mathrm{L}^{-1}$, the steady state biomass concentration is $200 ~\mathrm{mg} \mathrm{L}^{-1}$. The mean cell retention time in the reactor is $\_\_\_\_$ days. (answer in integer)
