A solution contains 61% of milk by volume. How many litres of this solution must be added to 31 litres of another solution, which contains 26% of milk, in order to produce a solution with overall concentration of milk 45%?

Using the rule of alligation, the ratio of the 61% solution to the 26% solution needed to reach a 45% concentration is (45 minus 26) to (61 minus 45), which is 19 to 16. If 16 parts equal 31 litres, then 1 part equals 1.9375 litres. The required volume of the 61% solution is 19 parts, which calculates to 36.8125 litres.

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