A 60-litre mixture contains milk and water in the ratio 2 : 1 (i.e., 40 L milk and 20 L water). How much water must be added so that the new ratio of milk to water becomes 1 : 2?

Introduction / Context:
This mixture problem requires adjusting the water quantity to achieve a target ratio. Milk quantity remains unchanged when only water is added; the ratio shift is achieved by increasing the water part.

Given Data / Assumptions:

• Total initial mixture = 60 L with milk : water = 2 : 1.
• Milk = 40 L; water = 20 L.
• Add x litres of water so that milk : water = 1 : 2.

Concept / Approach:
Set up the ratio equation with milk fixed at 40 and water as 20 + x. Enforce 40 : (20 + x) = 1 : 2, then solve for x.

Step-by-Step Solution:
40 / (20 + x) = 1 / 2. Cross-multiply: 40 * 2 = 20 + x. 80 = 20 + x ⇒ x = 60. Therefore, add 60 litres of water.

Verification / Alternative check:
New water = 20 + 60 = 80 L. New total = 120 L. Milk : water = 40 : 80 = 1 : 2, as desired.

Why Other Options Are Wrong:
42, 56, or 77 L do not yield a 1 : 2 ratio when substituted into 40 : (20 + x).

Common Pitfalls:
Misreading the initial ratio or thinking milk changes. Only water is added, so milk remains 40 L. Set the ratio correctly to avoid algebra errors.

Final Answer:
60 litres