Concrete mixer planning: With an effective working time of 7 hours and a 150-litre mixer taking 3 minutes per batch, estimate the practical output (litres) for the day, allowing a reasonable efficiency factor for loading/unloading losses.

Introduction / Context:
Estimating mixer output is essential for scheduling manpower, transit of materials, and pump availability. Rated drum capacity rarely equals delivered concrete per batch because of loading, discharge, and spillage inefficiencies. This problem uses simple production arithmetic with a practical efficiency factor.

Given Data / Assumptions:

• Mixer nominal capacity = 150 litres per batch.
• Cycle time = 3 minutes per batch (including mixing).
• Effective working time = 7 hours in the day.
• Adopt a reasonable delivery efficiency of about 0.90 of nominal (accounts for under-filling, sticking, delays).

Concept / Approach:
Total output equals number of batches multiplied by delivered volume per batch. The delivered volume per batch is less than the nominal capacity; a common planning allowance is 90% when crews and supply are well organized. The calculation proceeds stepwise from time to batches to litres.

Step-by-Step Solution:
Total minutes available = 7 hours * 60 = 420 minutes.Number of batches = 420 / 3 = 140 batches.Effective volume per batch = 0.90 * 150 = 135 litres.Daily output = 140 * 135 = 18,900 litres.

Verification / Alternative check:
Without the efficiency factor, theoretical output = 150 * 140 = 21,000 litres. Applying 90% aligns with typical site experience for small mixers considering charging/discharge losses and minor waiting times.

Why Other Options Are Wrong:
15,900–17,900 litres imply unrealistically low efficiency for a well-run crew, while values above 18,900 litres neglect normal losses.

Common Pitfalls:
Using drum capacity as delivered concrete; ignoring time lost in positioning, water dosing, and slump adjustment.

Final Answer:
18,900 litres.