Water–cement ratio calculation in mix design: If 1 standard bag of cement (50 kg) requires 30 litres of water for a particular mix, what is the water–cement ratio (by mass)?

Introduction / Context:
The water–cement ratio (w/c) is a key parameter controlling strength, durability, and workability of cement-based materials. It is defined as the mass of water divided by the mass of cement in the mix. This numerical question checks basic calculation and unit interpretation.

Given Data / Assumptions:

• Mass of cement in one bag = 50 kg.
• Water added = 30 litres.
• Assume density of water ≈ 1 kg per litre, so 30 litres ≈ 30 kg of water.

Concept / Approach:
Compute w/c = mass of water / mass of cement. Using consistent mass units yields a dimensionless ratio. Accurate unit handling avoids common mistakes (e.g., dividing by volume of cement rather than mass).

Step-by-Step Solution:

Verification / Alternative check:
Cross-check: 0.6 is a common w/c for moderate workability mixes; lower w/c (0.4–0.5) is used for higher strength/durability with plasticizers.

Why Other Options Are Wrong:

• 0.40 / 0.50: would correspond to 20 kg or 25 kg of water for 50 kg cement, not the given 30 kg.
• None of these: incorrect because 0.60 matches the calculation.

Common Pitfalls:

• Using litres directly without converting to mass for w/c.
• Assuming a 40 kg cement bag (some markets) instead of 50 kg; always confirm bag mass.

Final Answer:
0.60