How many cubic yards of concrete are needed for a rectangular slab that measures 40 feet by 60 feet with a thickness of 6 inches?
Correct Answer
B) 44.4 cubic yards
Calculate: 40' × 60' × 0.5' = 1200 cubic feet. Convert to cubic yards by dividing by 27: 1200 ÷ 27 = 44.4 cubic yards.
Why This Is the Correct Answer
Option A is correct because it follows the proper sequence of calculating concrete volume. First, convert all measurements to the same unit (6 inches = 0.5 feet), then multiply length × width × thickness to get cubic feet (40' × 60' × 0.5' = 1,200 cubic feet). Finally, convert to cubic yards by dividing by 27 (since there are 27 cubic feet in a cubic yard): 1,200 ÷ 27 = 44.4 cubic yards.
Why the Other Options Are Wrong
Option A: 400 cubic yards
This answer comes from calculating the area only (40 × 60 = 2,400) and then incorrectly dividing by 6 instead of properly calculating volume and converting to cubic yards.
Option C: 2400 cubic yards
This answer results from forgetting to convert the 6-inch thickness to feet, using 6 feet instead of 0.5 feet in the calculation (40 × 60 × 6 ÷ 27 = 133.3).
Memory Technique
Remember '27 to fly' - you need 27 cubic feet to make 1 cubic yard 'fly' off your calculation. Also, always convert inches to feet first by dividing by 12.
Reference Hint
Florida Building Code, Chapter 19 - Concrete, or construction mathematics reference sections covering volume calculations and unit conversions
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