A 60-liter mixture has milk and water in the ratio 2 : 1 (milk = 40 L, water = 20 L). How much additional water must be added so that the ratio becomes 1 : 2 (milk : water)?

Introduction / Context:
Only water is added to change the composition. Milk remains constant while water increases until the desired ratio is reached. This is a straightforward ratio adjustment.

Given Data / Assumptions:

• Total initial = 60 L; milk : water = 2 : 1 ⇒ milk = 40 L, water = 20 L.
• Add x liters of water; milk remains 40 L.
• Target milk : water = 1 : 2.

Concept / Approach:
Set up the ratio equation with constant milk and unknown water after addition, then solve for x.

Step-by-Step Solution:
After adding x, water = 20 + xRequire 40 : (20 + x) = 1 : 2Cross-multiply: 2*40 = 20 + x ⇒ 80 = 20 + x ⇒ x = 60

Verification / Alternative check:
New amounts: milk = 40 L; water = 80 L; ratio 40 : 80 = 1 : 2 as required.

Why Other Options Are Wrong:
20, 30, 40, and 50 do not yield water = 80 L. Only 60 L addition achieves the target ratio.

Common Pitfalls:
Changing milk as well or forming milk : total ratio instead of milk : water.

Final Answer:
60 litres