QUESTION $20$

A lake contains $2{10}^5{m}^3$ of water and is fed by a river discharging $9{10}^4\frac{m^3}{y}r.$ Evaporation across the lake surface removes $27\mathrm{.}4\frac{m^3}{?}$ day on average and the flow leaving the lake is about $8{10}^4\frac{m^3}{y}r.$ The upstream river is polluted with a chemical at a concentration of $6m\frac{g}{L}($assume the chemical does not ev$$porate with the water$).$ Inside the lake, the chemical decays at a rate of $0\mathrm{.}12y{r}^{-1}.$ Suddenly, the source of contamination to the River is removed. Determine what the concentration in the lake wquld be after one year.