The cost of 1 litre of milk is Rs. 20, what amount of water should be added to 1 litre of mixture to gain 25% profit, if the mixture is being sold at Rs. 20/litre?

Let’s denote the amount of water to be added as \(x\) litres.

Given:
– The cost of 1 litre of milk is Rs. 20.
– The selling price of the mixture is Rs. 20 per litre.
– The desired profit is 25%.

The cost price of 1 litre of milk is Rs. 20, and to achieve a 25% profit, the selling price should be:
\[ \text{Selling price} = \text{Cost price} + 25\% \times \text{Cost price} = 20 + 0.25 \times 20 = Rs. 25 \]

However, the mixture is being sold at Rs. 20 per litre. So, we need to find the volume of the mixture that can be sold for Rs. 25 to achieve the desired profit.

Since the selling price per litre is Rs. 20, to achieve a total selling price of Rs. 25, we need:
\[ \text{Volume of mixture} = \frac{\text{Total selling price}}{\text{Selling price per litre}} = \frac{25}{20} = 1.25 \text{ litres} \]

Therefore, the amount of water that should be added to 1 litre of milk to make the volume of the mixture 1.25 litres is:
\[ \text{Amount of water} = \text{Volume of mixture} – \text{Volume of milk} = 1.25 – 1 = 0.25 \text{ litres} \]

Conclusion

To gain a 25% profit by selling the mixture at Rs. 20 per litre, 0.25 litres of water should be added to 1 litre of milk.