(a ) Let the fixed amount be ₹ x and varying amount bo ₹ y per unit. According to the question, x + 540y = 1800 ...(i) x + 620y = 2040 ...(ii) On solving Eqs. (i) and (ii), we get x = 180, y = 3 ∴ Amount of bill for 500 units = 180+ 500 x 3 = ₹ 1680