In a 100 litre of mixture the ratio of milk and water is 6:4. How much milk must be added to the mixture in order to make the ratio 3 : 1?

Step 1: Understand the initial composition

• Total mixture = 100 litres
• Milk : Water = 6 : 4
• Total parts = 6 + 4 = 10
• Milk = (6/10) × 100 = 60 litres
• Water = (4/10) × 100 = 40 litres

Step 2: Let the amount of milk to be added be x litres

• New milk quantity = 60 + x
• Water remains unchanged = 40 litres
• New ratio (Milk : Water) = (60 + x) : 40

Step 3: Set up the equation using the desired ratio 3 : 1

(60 + x) / 40 = 3 / 1
⇒ 60 + x = 3 × 40
⇒ 60 + x = 120
⇒ x = 120 - 60 = 60

Answer: 60 litres

To make the ratio of milk to water 3 : 1, 60 litres of milk must be added to the mixture.

This is a classic example of a ratio adjustment problem. It demonstrates how the addition of one component while keeping the other constant affects the overall ratio. These questions are commonly seen in quantitative aptitude exams and help build logical reasoning skills.