Here it is:

**A tank contains 10 kg of sodium hydroxide in 1000 litres of water. Water is continuosly added to the tank at a rate of 5 litres/min so that the mixture is diluted evenly. At the same time, the solution flows out at the same rate. Initially, there were 10 grams of sodium hydroxide in every litre of water. How long will it take for the concentration of the solution to drop to 3.2 grams per litre?**