In what ratio must water be mixed with milk to gain 162/3\% on selling the mixture at cost price?

Answer: Option A

Let C.P. of 1 litre milk be Rs. 1
S.P. of 1 litre of mixture = Rs. 1, Gain = $$\frac{50}{3}$$ %
∴ C.P. of 1 litre of mixture = $$100\times \frac{3}{350}\times 1$$    = $$\frac{6}{7}$$
BY the rule of alligation, we have:

∴ Ration of water and milk = $$\frac{1}{7}$$ : $$\frac{6}{7}$$ = 1 : 6