Using the initial conditions of L$0=5\mathrm{.}25$ mg$/$L and D$0=2\mathrm{.}2$ mg$/$L$,$ coefficients of kd $=0\mathrm{.}4$ day$-1,$ kr $=0\mathrm{.}6$

day$-1,$ a DOS value of $7\mathrm{.}4$ mg$/$L$,$ and a Critical Time $($tc$)$ of $5$ days, calculate the $$Critical DO$$ days,

$($mg$/$L$,$ one decimal place$).$