A milk vendor has 2 cans of milk. The first contains 25% water and the rest milk. The second contains 50% water. How much milk should he mix from each of the containers so as to get 12 litres of milk such that the ratio of water to milk is 3 : 5?

Answer: Option B

Let the cost of 1 litre milk be Rs. 1
Milk in 1 litre mixture in 1^st can = $$\frac{3}{4}$$ litre, C.P. of 1 litre mixture in 1^st can Rs. $$\frac{3}{4}$$
Milk in 1 litre mixture in 2^nd can = $$\frac{1}{2}$$ litre, C.P. of 1 litre mixture in 2^nd can Rs. $$\frac{1}{2}$$
Milk in 1 litre of final mixture = $$\frac{5}{8}$$ litre, Mean price = Rs. $$\frac{5}{8}$$
By the rule of alligation, we have:

∴ Ratio of two mixtures = $$\frac{1}{8}$$ : $$\frac{1}{8}$$ = 1 : 1
So, quantity of mixture taken from each can = $$\left(\frac{1}{2}\times 12\right)$$  = 6 litres