A lake contains V $=2$ x$105$ m$3$ of water and is fed by a river discharging Qupstream $=9$ x$104$ m$3/$year$.$ Evaporation across the surface takes away Qevaporation $=1$ x$104$ m$3/$year$,$ so that only Qdownstream $=8$ x$104$ m$3/$year exits the lake in the downstream stretch of the river. The upstream river is polluted, with concentration C $=6\mathrm{.}0$ mg$/$L$.$ Inside the lake, this pollutant decays with rate k $=0\mathrm{.}12/$year$.$ Find the steady$-$state concentration of the pollutant in the downstream.

in this question isnt the reaction ratte negative and since its decaying so another negative sign the net sign will be positive