From the 50 litres of pure milk, 5 litres of milk is taken out and 5 litres water is added. This process is repeated 3 times, the amount of milk left after the 3rd replacement is:

36.45 Liters

Quantity of milk in the end 

= 50 ( 1 - \(\frac{5}{50}\) )³

= 50 ( 1 - \(\frac{1}{10}\) )³   

= 50 × \(\frac{9}{10}\) × \(\frac{9}{10}\) × \(\frac{9}{10}\)

= 36.45L