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 expression.