$.$A recent test has shown that monochloramine $(N{H}_{2}Cl)$ decays slowly

according to first$-$order kinetics. The results from the test showed a $20\%$

decay occurred over $8$ hours. Assume a drinking water treatment plant is

discharging effluent containing $2m\frac{g}{L}N{H}_{2}Cl$ into a river and that a $1$:$10$

dilution of this discharge is achieved in the river. If a concentration greater

than $0\mathrm{.}002m\frac{g}{L}N{H}_{2}Cl$ is harmful for the trout in the river, how many days

$($answer with $2$ decimal places$)$ will it take for the receiving river water to

become "safe" for the trout?