This is a question from first Order differential equation

5.12. The Melting Pot. A tank containing 20,000 litre of blended fuel oil is stirred contin- uously to keep the mixture homogeneous. The fuel is blended from a heavy and a light component in equal proportions. The two components are added at a rate of 5 litres/min each, then the blended fuel oil is withdrawn at a rate 10 litres/min to be burnt in a furnace. The blended fuel oil will not burn in the furnace if the heaV)r component forms more than 80% of the mixture. At a certain instant the supply of the light component ceases whilst the heavy com- ponent continues to be added at the same rate of 5 litres/ min and the homogeneous mixture is withdrawn at the same rate of 10 litres/ min. A (a) Let a: be the amount of heavy fuel in the tank (in litres) at time 1' minutes aft 26/46 V the supplyr of light oil ceases. Show that \\ do: 10:15 _ =