How much water be added to a bucket which contains 40 L of milk at the cost price of Rs. 3.50 per litre, so that the cost of milk reduced to Rs 2 per litre ?

To solve this problem, we need to understand that the cost of milk per litre is reduced when water is added because the total volume increases while the total cost remains the same.

Here are the steps to solve this problem:

1. The total cost of the milk in the bucket is 40 L * Rs. 3.50/L = Rs. 140.

2. We want the cost of the milk to be reduced to Rs. 2 per litre. This means that the total cost of Rs. 140 should be spread over a larger volume.

3. Let's denote the volume of water to be added as 'x' litres. So, the total volume after adding the water is 40 L + x L.

4. The cost per litre after adding the water is the total cost divided by the total volume, which is Rs. 140 / (40 L + x L) = Rs. 2/L.

5. Solving the equation 140 / (40 + x) = 2 for 'x' gives x = 30 L.

So, 30 litres of water should be added to the bucket to reduce the cost of milk to Rs. 2 per litre.