Explanation:

Let the milk he bought is 1000 ml

Let C.P of 1000 ml is Rs. 100

Here let he is mixing K ml of water

He is getting 30% profit

= Now, the selling price is also Rs. 100 for 1000 ml

= 100 : K%

= 100 : 30

10 : 3 is ratio of milk to water

= Percentage of milk = 10 x 100/13 = 1000/13 = 76.92%

Hence, option 3 is the correct answer.