In a 100 litre mixture, the ratio of milk to water is 6:4 (milk:water). How many litres of milk must be added to this mixture so that the new ratio of milk to water becomes 3:1 (milk:water), assuming only milk is added and the water quantity remains unchanged?

Introduction / Context:
This question checks ratio adjustment by adding one component. When only milk is added, the water quantity stays constant, and only the milk part increases. You can compute the initial milk and water in litres from the ratio, then apply the target ratio equation to find how much milk must be added.

Given Data / Assumptions:

• Total mixture initially = 100 L
• Initial milk:water = 6:4
• Only milk is added
• Final milk:water required = 3:1

Concept / Approach:
Convert ratio to actual quantities using total. Then enforce (milk + added)/(water) = 3/1.

Step-by-Step Solution:

Verification / Alternative check:
After adding 60 L milk, milk = 120 L and water = 40 L. Ratio = 120:40 = 3:1. Verified exactly.

Why Other Options Are Wrong:

Common Pitfalls:
Some students mistakenly change water as well, or assume the final total must remain 100 L, which is incorrect because you are adding milk. Another mistake is converting ratio 3:1 into “add 2 parts milk,” which ignores the actual litre values present.

Final Answer:
60 litres