Neco 2025 Chemistry Question 41

A factory releases an effluent that contains \(30\) mg/L nitrate \(\text{NO}_3^-\) and \(5\) mg/L phosphate \(\text{PO}_4^{3-}\). The effluent flow rate is \(10{,}000\) L·h\(^{-1}\) and mixes with river water flowing at \(20{,}000\) L·h\(^{-1}\). The water‑quality standards are \(10\) mg/L for nitrate and \(0.5\) mg/L for phosphate. The factory can treat its effluent to reduce both contaminants by the same percentage \(r\) (\%). Determine the minimum value of \(r\) that will make the concentrations in the mixed river water meet the standards. The treatment cost is \(\$0.0002\) per litre of effluent for each percent reduction. Calculate the minimum monthly cost (30 days) of the required treatment.