The price of high grade milk is ₹ 40 per litre and that of low grade milk is ₹ 30 per litre. In what ratio they should be mixed so that the resultant mixture has a price of ₹ 36 per litre?

To solve this problem, we can use the method of allegation which is a rule of mixture to find the ratio in which two or more ingredients at the given price must be mixed to produce a mixture of a desired price.

Step 1: Write down the prices of the two types of milk. High grade milk is ₹40 per litre and low grade milk is ₹30 per litre.

Step 2: Write down the price of the mixture, which is ₹36 per litre.

Step 3: Calculate the differences between the prices of the high grade and low grade milk and the price of the mixture. The difference between the price of the high grade milk and the mixture is ₹40 - ₹36 = ₹4. The difference between the price of the low grade milk and the mixture is ₹36 - ₹30 = ₹6.

Step 4: The ratio in which the two types of milk should be mixed is the inverse of the differences calculated in step 3. So, the high grade milk and low grade milk should be mixed in the ratio of 6 : 4 or 3 : 2.

So, the high grade milk and low grade milk should be mixed in the ratio of 3 : 2 to get a mixture that costs ₹36 per litre.