In what ratio must water be mixed with milk to gain $$16\frac{2}{3}\%$$ on selling the mixture at cost price?

Let the cost price of 1 litre of milk be C

Now, assume that we mix x litres of milk with y litres of water.

The total cost price of the mixture = C × x (water is free).

The selling price must be 16.67% more than the cost price, i.e. 7/6 of the cost price

For the mixture to sell at the cost price per liter, the selling price should be equal to the cost price per liter.

Solving the equation, we get, x : y = 6 : 1