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Correct answer: 19:6
The 84-litre mixture in the ratio $5:2$ has milk $=60$ litres and water $=24$ litres. Removing 21 litres removes milk and water in the same ratio, so 15 litres milk and 6 litres water are removed; then 12 litres of milk is added. Thus milk becomes 57 litres and water remains 18 litres, giving the ratio $57:18=19:6$.