Looking for a detailed explanation please q33 practice

4. i) A tank can hold 1000 litres. Initially it holds 250 litres of pure water. Brine, which contains 5 grams of salt per litre, is run into the tank at the rate of 6 litres per minute. The mixture, which is stirred continuously, is run off at a rate of 1 litre per minute. Let M(t) denote the mass of salt (measured in grams) present in the tank after & minutes, where 0