Four complete$-$mix flow reactors $($CMFRs$)$ are used in series to remove COD from

wastewater. The hydraulic detention time for each reactor is $=2\mathrm{.}5$ hours. The influent COD

concentration is $C_0=100m\frac{g}{L}.$ If the reaction is first order with a rate constant of $k=2\mathrm{.}0hr$

$-1,$ what will be the COD concentration $C$ in the effluent from the fourth reactor? Round your

answer to $1$ digit past the decimal $($e$.$g$.,$ XX$.$X$)$ and do NOT enter units.