To 100 litres of milk, 10 litres of water is added and then 20 litres

After 10 liters of water are added to 100 liters of milk, we have a 110-liter solution with the milk-to-water ratio being 10 : 1. After 20 liters of solution are removed, there are 90 liters left. Since the milk to water ratio at this point is still 10 : 1, we can set up the equation:

10x + x = 90

11x = 90

x = 90/11

Therefore, there are 900/11 liters of milk and 90/11 liters of water in the 90-liter solution.

Now, after 30 liters of water are added to the 90-liter solution, we have a 120-liter solution, of which 900/11 liters is milk and 90/11 + 30 = 90/11 + 330/11 = 420/11 liters is water. So the milk-to-water ratio is 900/11 : 420/11 = 900 : 420 = 15 : 7.

Finally, after 20 liters of solution are removed, we have a 100-liter solution left. Since the milk to water ratio at this point is still 15 : 7, we can set up the equation:

15y + 7y = 100

22y = 100

y = 100/22 = 50/11

Therefore, there are 15(50/11) = 750/11 liters of milk (and 7(50/11) = 350/11 liters of water) in the 100-liter solution.

Answer: D