In what ratio of water be mixed with milk to gain 16 2/ 3 % on selling the mixture at cost price?

Let C.P. of 1 liter milk be Re. 1, Gain = 16 ²/₃ % = 50/3 %
and S.P. of 1 liter mixture = Re. 1
then C.P. of 1 liter mixture = (1 x (100 x 3) / 350) = Re. (6 / 7)
By the rule of alligation,
Hence, required ratio = (1/ 7) : (6 / 7) = 1 : 6