A concrete slab measures 30 feet by 40 feet with a thickness of 6 inches. How many cubic yards of concrete are needed?
Correct Answer
C) 22.2 cubic yards
Volume = 30' × 40' × 0.5' = 600 cubic feet. Converting to cubic yards: 600 ÷ 27 = 22.2 cubic yards. Always convert thickness to feet (6 inches = 0.5 feet) before calculating.
Why This Is the Correct Answer
Option B is correct because the calculation properly converts all measurements to the same unit (feet) before computing volume. The thickness of 6 inches must be converted to 0.5 feet, then multiplied by length and width to get 600 cubic feet. Finally, dividing by 27 (since there are 27 cubic feet in a cubic yard) gives 22.2 cubic yards. This follows the standard concrete volume calculation procedure used in construction.
Why the Other Options Are Wrong
Option A: 24.7 cubic yards
This answer likely results from an error in unit conversion or calculation. It's significantly lower than the correct answer, possibly from incorrectly converting the thickness or making an arithmetic error in the final division.
Option B: 27.0 cubic yards
This answer suggests the calculation used 6 inches as 6 feet instead of converting to 0.5 feet, or another significant unit conversion error that inflated the final result.
Memory Technique
Remember 'TWENTY-SEVEN' - the number of cubic feet in a yard. For thickness conversion, think 'Twelve Inches = One Foot' so divide inches by 12.
Reference Hint
Look up concrete calculations and volume conversions in the construction materials chapter, specifically the section on concrete quantity takeoffs and unit conversions.
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