A drum contains 80 litres of ethanol. 20 litres of this liquid is removed and replaced with water. 20 litres of this mixture is again removed and replaced with water. How much water (in litres) is present in this drum now?

We know the formula:

X = A(1 - R/C)n

Here X = Liquid remaining after replacement

A = Total quantity of liquid before replacement

R = Quantity of replaced liquid

C = Total Capacity of drum

n = No. of times the liquid was replaced

⇒ A = 80, R = 20, C = 80 and n = 2

Putting these values in the formula,

⇒ X = 80 × (1 - 1/4)2

⇒ X = 80 × 9/16

⇒ X = 45 litres

⇒ Amount of ethanol present after replacement = 45 litres

∴ Amount of water present after replacement = 80 - 45 = 35 litres

C is correct choice.