Explanation:

- Start with 30 liters of single refined oil, which has 20% impurities.

- This means 6 liters are impurities (20% of 30 liters) and 24 liters are pure oil (80% is pure oil).

- Double refining reduces impurities from 20% to 4%, keeping the same amount of pure oil.

- With 24 liters of pure oil, double refining allows for a total volume where 4% is impurities.

- Calculate the final volume (V): $V\times 0.96=24$ liters.

- Solving gives $V=25$ liters.

- Option 1: 24.0 liters - too low.

- Option 2: 24.8 liters - too low.

- Option 3: 25.2 liters - slightly too high.

- Option 4: 25.5 liters - too high.

- Correct Answer: 25.0 liters.

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