Ajay purchased two different kinds of vinegar. In the first mixture, vinegar is 75% and the second mixture, vinegar is 83.33%. If he mixes the two given mixtures and makes a third mixture in which the percentage of vinegar is 80%, what is the quantity of the first mixture that is required to make 20 litres of the third kind of mixture?

Using alligation: First mixture (75% vinegar) and second mixture (83.33% vinegar) combine to make 80% vinegar. Representing 83.33% as 5/6, the alligation diagram shows the ratio of first to second mixture is 2:3. For 20 litres total, first mixture = (2/5) × 20 = 8 litres. Verification: 8L at 75% + 12L at 83.33% = 6L + 10L = 16L vinegar in 20L total = 80%.