A completely mixed retention basin is used to treat an industrial waste water consisting of a non-conservative pollutant, using a reaction that destroys the pollutant according to first-order reaction kinetics, with k = 0.216/day. The volume of the basin is 500 m^3, the volumetric flow rate of the industrial waste water in and out of the basin is 50 m^3/day, and the inlet pollutant concentration is 100 mg/L. What is the outlet concentration after the treatment?