When estimating concrete work, a contractor calculates 150 cubic yards are needed. Applying a 10% waste factor and 5% overage allowance, what quantity should be ordered?
Correct Answer
C) 173 cubic yards
Calculate: 150 cubic yards × 1.10 (waste factor) × 1.05 (overage allowance) = 150 × 1.155 = 173.25, rounded to 173 cubic yards.
Why This Is the Correct Answer
Option C correctly applies both the 10% waste factor and 5% overage allowance as multiplicative factors. The calculation is: 150 cubic yards × 1.10 (waste factor) × 1.05 (overage allowance) = 150 × 1.155 = 173.25 cubic yards. Since concrete is typically ordered in whole cubic yards, this rounds to 173 cubic yards, ensuring adequate material for the project.
Why the Other Options Are Wrong
Option A: 165 cubic yards
Option D of 180 cubic yards significantly overestimates the required concrete. This might result from incorrectly calculating 150 × 1.20 (adding 20% total) or applying the factors incorrectly. While having extra material seems safer, over-ordering increases project costs unnecessarily and violates proper estimating practices.
Option B: 172.5 cubic yards
Option B represents the intermediate calculation of 150 × 1.15 = 172.5 cubic yards, which incorrectly adds the waste factor and overage allowance as percentages rather than applying them as separate multiplicative factors. This additive approach underestimates the total concrete needed.
Option D: 180 cubic yards
Option A incorrectly adds the percentages (10% + 5% = 15%) instead of multiplying the factors. This gives 150 × 1.15 = 172.5, then rounds down to 165, which significantly underestimates the required concrete quantity and could lead to material shortages on the job site.
Memory Technique
Remember 'Multiply, Don't Add' - waste and overage factors are multiplicative: 1.10 × 1.05 = 1.155. Think 'WOM' - Waste times Overage times Material.
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