A mixture contains 25% milk and rest water. What percent of this mixture must taken out and replaced with milk so that in mixture milk and water may become equal.

Let the total mixture be 100 litres for simplicity.

• Milk = 25% of 100 = 25 litres
• Water = 100 - 25 = 75 litres

Let x litres of the mixture be taken out and replaced with milk.

When x litres is removed, it removes milk and water in the same 25:75 ratio i.e., 1:3.

• Milk removed = (1/4) × x = x/4
• Water removed = (3/4) × x = (3x)/4

After removing x litres:

• Milk left = 25 - x/4
• Water left = 75 - 3x/4

We now add x litres of pure milk, so:

• New milk quantity = 25 - x/4 + x = 25 + (3x)/4
• Water remains = 75 - (3x)/4

We want milk and water to be equal:

25 + (3x)/4 = 75 - (3x)/4
Add (3x)/4 to both sides:
25 + (3x)/4 + (3x)/4 = 75
25 + (6x)/4 = 75
(6x)/4 = 50
6x = 200
x = 33.33

So, 33.33 litres out of 100 litres must be replaced.

Percentage to be replaced = (33.33 / 100) × 100 = 33.33%

Answer: 33.33%