Explanation:

We are given:

Single refined oil contains 20% impurities → 80% pure oil.

After double-refining, the oil contains 4% impurities → 96% pure oil.

We start with 30 litres of single refined oil.

Step 1: Find how much pure oil is present in 30 litres of single refined oil.
Since it contains 80% pure oil:

Pure oil = 30 × 80/100 = 24 litres

Step 2: Let the amount of double-refined oil be ?? lliters

Since it contains 96% pure oil, we have:

Pure oil in ?? litres of double-refined oil
= ?? × 96/100 = 0.96

We know the pure oil remains the same, i.e., 24 litres. So:

0.96?? = 24
⇒ ?? = 24/0.96
= 25
Final Answer: 25 litres of double-refined oil can be obtained.