Prabhat wants to invest the total amount of Rs 15,000 in saving certificates and national saving bonds. According to rules, he has to invest at least Rs 2000 in saving certificates and Rs 2500 in national saving bonds. The interest rate is 8 % on saving certificate and 10 % on national saving bonds per annum. He invest Rs x in saving certificate and Rs y in national saving bonds. Then the objective function for this