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.