Correct Answer - Option 4 : 20 litres

**Given:**

Total initial mixture = 80 litres

The percentage of petrol in the mixture is 25%

**Calculation:**

The quantity of petrol in initial mixture = 25/100 × 80 = 20 litres

The quantity of ethanol in initial mixture = 80 - 20 = 60 litres

The amount of ethanol added be x

According to the question;

⇒ 20/(60 + x) = 20/80

⇒ 80 = 60 + x

⇒ x = 20 litres

**∴ The amount of ethanol added is 20 litres**