Question : 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?

Option 1: 45

Option 2: 40

Option 3: 35

Option 4: 44

Correct Answer: 35

Solution : Total volume of mixture = 80 litres
Liquid replaced with water is 20 litres and this process is repeated two times.
Pure ethanol left in the mixture after that
$={\text{total mixture}×(\frac{(\text{total mixture – liquid removed}}{\text{total mixture}})^{\text{no of times liquid is replaced}}}$
$= 80 × (\frac{80-20}{80})^2$
$= 80 × \frac{9}{16}$
$= 45$ litres
So, water present in the final solution = 80 – 45 = 35 litres
Hence, the correct answer is 35.