In what ratio must water be mixed with milk to gain 16(2/3)% on selling the mixture at cost price? Choose the correct choice from the following options.
A) 1 : 6  B) 6 : 1  C) 2 : 3  D) 4 : 3  E) 3 : 2

Rationale: "explanation : let c . p . of 1 litre milk be re . 1 . s . p . of 1 litre of mixture = re . 1 , gain = 50 / 3 % c . p . of 1 litre of mixture = ( 100 x 3 / 350 x 1 ) = 6 / 7 by the rule of alligation , we have : ratio of water and milk = 1 / 7 : 6 / 7 = 1 : 6 . answer is a"
Answer: A