Suppose a container contains A units of a liquid from which B units are taken out and replaced by water. After n operations, Quantity of pure liquid in the final mixture =A[1−(B∕A)]n Calculation: By using the above formula, the quantity of pure milk in the final mixture ⇒40×[1−(4∕40)]3 ⇒40×[1−(1∕10)]3 ⇒40×(729∕1000)=29.16 ∴ The quantity of milk in the final mixture is 29.16 litres.