$(10$ pts$)$ If the influence to an aeration tank has BOD$5=200$ mg$/$L$,$ effluent BOD$5=10$ mg$/$L$,$ MLSS $=3000$mg$/$L$,$ Y $=0\mathrm{.}8$lb MLSS$/$lb BOD$5,$ ke$=0\mathrm{.}03$day$^-1,$ aeration time $($t$)=7$ hr$,$ What is the mean cell residence time, c$?$