Adjusting milk:water ratio by adding water: A 729-ml mixture has milk:water = 7 : 2. How much additional water must be added to make the ratio 7 : 3?

Introduction / Context:
Ratios convert to absolute amounts when total volume is known. Compute current milk and water, then find the target water amount given a fixed milk quantity and new target ratio, and finally the extra water required.

Given Data / Assumptions:

• Total = 729 ml; milk:water = 7 : 2 ⇒ total parts = 9.
• Milk = 7/9 * 729 = 567 ml.
• Current water = 729 − 567 = 162 ml.
• Target ratio milk:water = 7 : 3 with the same milk amount (no milk added).

Concept / Approach:
For ratio 7 : 3 and milk = 567 ml, water must be (3/7)*567 = 243 ml. The additional water is target minus current.

Step-by-Step Solution:

Verification / Alternative check:
New total = 567 + 243 = 810 ml; ratio 567:243 reduces to 7:3, correct.

Why Other Options Are Wrong:
Offered numerical choices (600, 710, 520) are far from the computed 81 ml; hence “None of these” is correct for the provided list.

Common Pitfalls:
Trying to maintain total volume 729 ml; note that adding water increases total volume but does not change milk amount.

Final Answer:
None of these