When one litre of water is added to a mixture of milk and water, the new mixture contains 25% of milk. When one litre of milk is added to the new mixture, then the resulting mixture contains 40% milk. What is the percentage of milk in the original mixture?

Let's denote:

• M as the amount of milk in the original mixture
• W as the amount of water in the original mixture

From the problem, we know that:

1. When one litre of water is added to the mixture, the new mixture contains 25% of milk. This can be written as: M / (M + W + 1) = 0.25
2. When one litre of milk is added to the new mixture, the resulting mixture contains 40% milk. This can be written as: (M + 1) / (M + W + 2) = 0.40

We now have a system of two equations that we can solve to find the values of M and W.

Solving the first equation for M gives: M = 0.25 * (M + W + 1)

Solving the second equation for M gives: M = 0.40 * (M + W + 2) - 1

Setting these two equations equal to each other gives: 0.25 * (M + W + 1) = 0.40 * (M + W + 2) - 1

Solving this equation for W gives: W = 3

Substituting W = 3 into the first equation gives: M = 0.25 * (M + 3 + 1)

Solving this equation for M gives: M = 1

So, the original mixture contained 1 litre of milk and 3 litres of water, which means the percentage of milk in the original mixture was 1 / (1 + 3) = 25%.