In wastewater treatment, organism biomass increases as pollutants are taken up and metabolized. This increase is reflected in the amount of sludge generated at the wastewater$-$treatment plant, a residue that must receive safe disposal. Engineers use the yield coefficient $($Y$)$ to calculate biomass $($sludge$)$ production. Laboratory studies have shown that microorganisms produce $10$ mg$/$L of biomass in reducing the concentration of a pollutant by $50$ mg$/$L$.$ Calculate the yield coefficient, specifying the units of expresion.