A concrete slab measures 40 feet by 60 feet and is 6 inches thick. How many cubic yards of concrete are needed?
Correct Answer
A) 44.4 cubic yards
Volume = 40' × 60' × 0.5' = 1,200 cubic feet. Convert to cubic yards by dividing by 27: 1,200 ÷ 27 = 44.4 cubic yards. Always remember to convert thickness to feet (6 inches = 0.5 feet) and divide by 27 to get cubic yards.
Why This Is the Correct Answer
Option A is correct because the calculation properly converts all measurements to consistent units (feet), calculates the volume in cubic feet, and then converts to cubic yards. The thickness of 6 inches must be converted to 0.5 feet before multiplying. The final step divides by 27 (since there are 27 cubic feet in one cubic yard) to get 44.4 cubic yards.
Why the Other Options Are Wrong
Option B: 133.3 cubic yards
This answer likely results from forgetting to convert the 6-inch thickness to feet, instead using 6 as the thickness value, which would give an incorrect volume calculation that's three times too large.
Option C: 400 cubic yards
This answer represents the area in square feet (40 × 60 = 2,400) but appears to be a miscalculation or represents forgetting to include the thickness dimension entirely in the volume calculation.
Option D: 1,200 cubic yards
This answer represents the volume in cubic feet (1,200) without converting to cubic yards, showing the student forgot the final conversion step of dividing by 27.
Memory Technique
Remember '27 to the sky' - there are 27 cubic feet in a cubic yard (3×3×3=27). Always convert inches to feet by dividing by 12, then multiply L×W×H, then divide by 27 for cubic yards.
Reference Hint
Look up concrete volume calculations in the construction materials chapter, specifically the section on unit conversions and cubic yard calculations.
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