Milk–water adjustment: A 60 L mixture has milk and water in the ratio 2 : 1 (milk 40 L, water 20 L). How many litres of water must be added so that the final ratio becomes 1 : 2 (milk : water)? Show the proportion setup explicitly.

Introduction / Context:
Ratio-change problems in mixtures require keeping one component fixed if it is not removed or added. Here only water is added, so milk stays constant while water increases until the target ratio is reached.

Given Data / Assumptions:

• Total mixture = 60 L with milk : water = 2 : 1.
• Milk = 40 L; Water = 20 L.
• Add y litres of water; milk remains 40 L.
• Target ratio milk : water = 1 : 2.

Concept / Approach:
Use the target ratio 40 : (20 + y) = 1 : 2. Cross-multiply to solve for y while keeping milk constant because only water is added.

Step-by-Step Solution:

Verification / Alternative check:
New water = 20 + 60 = 80 L; milk = 40 L. Ratio = 40 : 80 = 1 : 2, correct.

Why Other Options Are Wrong:
20 L, 30 L, 40 L yield water totals 40, 50, 60 which produce ratios 40:40, 40:50, 40:60, none equal to 1:2; 50 L is also incorrect.

Common Pitfalls:
Recomputing milk incorrectly or trying to scale both parts without acknowledging that milk volume does not change when only water is added.

Final Answer:
60 L