How much water (in litres) must be added to 80 litres solution of milk and water containing 10% milk, so that it becomes a 5% milk solution?

Step 1: Understand the initial setup

• Total mixture = 80 litres
• Milk = 10% of 80 = 8 litres
• Water = 80 - 8 = 72 litres

Step 2: Let x litres of water be added

• Milk remains the same = 8 litres
• Water becomes = 72 + x litres
• New total solution = 80 + x litres

Step 3: Set up the equation

We want milk to be 5% of the new solution:

Milk / (Milk + Water) = 5 / 100
=> 8 / (80 + x) = 1 / 20

Step 4: Solve the equation

8 × 20 = 80 + x
160 = 80 + x
x = 160 - 80
x = 80

Answer: 80 litres

You need to add 80 litres of water to make the solution 5% milk.