A vessel contains 40 litres of orange juice. 4 litres of orange juice in the vessel was replaced by water. This process was again repeated twice with the mixture. How much orange juice is there in the final mix?

First 4L orange juice was removed
So now in 40L mixture, there is 40 - 4 = 36 litres orange juice and 4 litres water
Now, remove 4L mixture. While doing this proportionate amount of juice and water gets removed.
Amount of juice removed = 4 × (Juice quantity/Mixture quantity)
= 4 × (36/40)
= 3.6 Litres.

Orange Juice remaining = 36-3.6 = 32.4 Litres
Again 4L mixture removed
Amount of juice removed = 4 × (Juice quantity/Mixture quantity)
= 4 × (32.4/40)
= 3.24 Litres.

Juice remaining = 32.4 - 3.24
= 29.16 Litres.