A dishonest milkman professes to sell his milk at cost price but he mixes it with water and thereby gains 25%. The percentage of water in the mixture is:

Answer: Option C

Let C.P. of 1 litre milk be Rs. 1
Then, S.P. of 1 litre of mixture = Rs. 1, Gain = 25%
C.P. of 1 litre mixture = Rs. $$\left(\frac{100}{125}\times 1\right)$$  = $$\frac{4}{5}$$
By the rule of alligation, we have:

∴ Ratio of milk to water = $$\frac{4}{5}$$ : $$\frac{1}{5}$$ = 4 : 1
Hence, percentage of water in the mixture = $$\left(\frac{1}{5}\times 100\right)$$  % = 20%