$($Exam level problem$).$ Due to the input of phosphorus into Lake Scariett $($well mixed

volume of $8{10}^7{m}^3,$ water inflow $=$ water outflow $=3{10}^6\frac{m^3}{?}$ day$),$ an algal bloom

has formed. The algae generate a toxin within the lake at a rate of $50k\frac{g}{?}$ day. Sunlight

degrades the toxin with a rate constant of $0\mathrm{.}05h^{-1}.$