A concrete slab measures 40 feet by 60 feet with a thickness of 6 inches. 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: 1,200 ÷ 27 = 44.4 cubic yards.
Why This Is the Correct Answer
Option A is correct because the calculation properly converts all measurements to the same units (feet), calculates the volume in cubic feet, 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 (3×3×3=27).
Why the Other Options Are Wrong
Option B: 14,400 cubic yards
This answer likely results from forgetting to convert the 6-inch thickness to feet, instead using 6 as if it were feet: 40×60×6=14,400 cubic feet, then dividing by 108 instead of 27, or making another unit conversion error.
Option C: 1,200 cubic yards
This results from using 6 instead of 0.5 for the thickness, treating inches as feet: 40×60×6=14,400, and then not converting from cubic feet to cubic yards.
Memory Technique
Remember '27 to get away' - divide by 27 cubic feet to get cubic yards. For thickness, think 'dozen inches make a foot' - divide inches by 12.
Reference Hint
Look up concrete calculations and unit conversions in the construction materials chapter, specifically volume calculations and cubic yard conversions.
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