To start off this question let us assume that cost price of 1 litre milk is Rs 1
Now need to make a mixture and sell this mixture at 1 Rs per litre such that the total gain on mixture is 16.667%.
Therefore, CP of 1 litre of mixture becomes (quantity of milk)/ (quantity of mixture containing 1 L milk)×(price of 1 litre milk).
=>(100/(100+50/3)) × 1
=>CP of 1 litre milk of mixture: Rs 5/6
As price of any amount of water is zero, and as 1 litre milk costs Rs 1.5/6 litre
of mixture will comprise entirely of cost of milk which means,1 litre of mixture will
contain 5/6th amount of milk.
=>Water is added in the ratio of (1-5/6)=1/6