Mixture A has milk and water in the ratio $3 : 1$ and mixture B has them in the ratio $1 : 1$. How many litres of A must be mixed with 18 litres of B to get a mixture with milk and water in the ratio $2 : 1$?

Explanation

1. 01

   Milk fractions: A = $\frac{3}{4}$, B = $\frac{1}{2}$, target = $\frac{2}{3}$.

2. 02

   $A : B = \left(\frac{2}{3} - \frac{1}{2}\right) : \left(\frac{3}{4} - \frac{2}{3}\right) = \frac{1}{6} : \frac{1}{12} = 2 : 1$.

3. 03

   A = $2 \times 18 = 36$ litres.