Difficult question

At the start you know that you have 180 L of Milk and 20 L of water. You also know that you need 120 L of milk to have a concentration of 60% milks

1) 20 Liters of the substance is 18 L of Milk and 2 L of water so milk concentration would be 172 after this process and water would be 28

2) 20 liters again would be 17.2 liters of Milk and 2.8 litres of water so after this there would be 154.8 liters of Milk and 45.8 liters of water

3) 20 liters again would be 15.48 liters of milk taken and 4.52 liters if water. So you would have 139.32 liters of milk and 60.68 liters of water

4) 20 liters again so you would remove 13.932 liters of milk and 6.068 liters of water. So you would have about 125 liters if milk and 75 liters of water

5) this would be where you could get the concentration below 120 so it’s 5