An ideal complete$-$mix flow reactor $($CMFR$)$ is used to remove COD from wastewater.

The influent COD concentration is $C_0=100m\frac{g}{L}.$ If the reaction is first order with a

rate constant of $k=2\mathrm{.}0h{r}^{-1},$ what detention time $(r)$ in hours is required to achieve

an effluent COD concentration of $C=5m\frac{g}{L}?$ Round your answer to $1$ digit past the decimal