For a BOD test carried out in the laboratory, 5ml of wastewater having no initial DO was taken in a 300ml BOD bottle and mixed with dilution water of 9.2 mg/l DO concentration. After incubating the bottle for 5 days at 20^oC, the DO of the mixture was found to be 5.0mg/l. Compute the BOD₅(at 20^oC) of wastewater.