two streams A and stream B converges and make the stream C$.$ The flow rates of streams A and B are $3\mathrm{.}7$ m$3/$s and $2\mathrm{.}5$ and dissolved oxygen $($DO$)$ of streams A and B are $4\mathrm{.}5$ mg$/$L and $7\mathrm{.}5$ mg$/$lL$.$ The DO at stream C is $?$