Let the cost of 1 litre milk be Re. 1

Milk in 1 litre mix. in 1^st can = \(\frac{3}{4}\)  litre, C.P. of 1 litre mix. in 1^st can Re   \(\frac{3}{4}\)

Milk in 1 litre mix. in 2^nd can = \(\frac{1}{2}\) litre, C.P. of 1 litre mix. in 2^nd can Re   \(\frac{1}{2}\)

Milk in 1 litre of final mix.= \(\frac{5}{8}\)    litre, Mean price = Re    \(\frac{5}{8}\)

By the rule of alligation, we have:

C.P. of 1 litre mixture in 1^st can  =  \(\frac{3}{4}\)

C.P. of mixture in 2^nd can  =  \(\frac{1}{2}\)

Main price  =  \(\frac{5}{8}\)

So, Ratio of two mixtures = \(\frac{1}{8}:\frac{1}{8}=1:1\)

So, quantity of mixture taken from each can = \(\left(\frac{1}{2}\times12\right)= 6 litres\)