The two$-$pond system shown in the figure below is fed by a stream with flow rate of

$2\mathrm{.}5$ million gallons per day $($MGD$)$ and a BOD $($a nonconservative pollutant$)$ concentration $12$ mg$/$L$.$ The rate of decay of BOD is $0\mathrm{.}43/$day$.$ The volume of the first pond is $6\mathrm{.}5$ million gallons $($MG$),$ and the second is $3\mathrm{.}5$ MG$.$ Assuming complete mixing within each pond, find the BOD concentration leaving each pond.