A bottle of whisky contains 40% alcohol. If we replace a part of this whisky by another whisky containing 20% alcohol, the percentage of alcohol becomes 28%. What quantity of whisky is replaced?

By the method of alligation, the ratio of the quantity of the first whisky to the replaced second whisky is (28 minus 20) to (40 minus 28), which simplifies to 8 to 12 or 2 to 3. This means that in every 5 parts of the total mixture, 3 parts consist of the second whisky and 2 parts consist of the first whisky. The fraction of the total whisky that was replaced by the second variety is therefore 3 divided by 5.

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