18 litres of pure water was added to a vessel containing 80 litres of pure milk. 49 litres of the resultant mixture was then sold and some more quantity of pure milk and pure water was added to the vessel in the respective ratio of 2:1. If the resultant respective ratio of milk and water in the vessel was 4:1, what was the quantity of pure milk added in the vessel ? (in litres)

18 litres of pure water was added to 80 litres of pure milk.

This we get a mixture where
quantity of water = 18 litre
quantity of milk = 80 litre
Total quantity of the mixture = 18+80 = 98 litre

49 litres of the resultant mixture was then sold. Since half of the mixture is removed and only the other half is remaining,
quantity of water remaining = 18/2 = 9 litre
Quantity of milk remaining = 80/2 = 40 litre
Total quantity remaining = 49 litre

some more quantity of pure milk and pure water was added to the vessel in the ratio 2:1
Let quantity of milk added =2x
quantity of water added =x

Now,
quantity of water =9+x
quantity of milk =40+2x

Given that ratio of milk and water in the vessel is now 4:1
=> (40+2x):(9+x)=4:1
=> 40+2x=4(9+x)
=> 40+2x=36+4x
=> 2x=4
=> x=2

quantity of pure milk added in the vessel =2x = 4 litre