Two batch reactors are used in series to treat industrial wastewater. The average COD concentration in the influent is $500$ mg$/$L$.$ The retention time in reactor A is $3$h$,$ while the retention time in reactor B is $2$h$.$ The removal of COD is described by a first$-$order reaction in both reactors with a reaction constant of $0\mathrm{.}8$ h$-1.$ Determine the COD concentration in the effluent of reactor B$.$