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. Converting to cubic yards: 1,200 ÷ 27 = 44.4 cubic yards. Remember there are 27 cubic feet in one cubic yard.
Why This Is the Correct Answer
The correct answer is A because we must first calculate the volume in cubic feet (40' × 60' × 0.5' = 1,200 cubic feet), then convert to cubic yards by dividing by 27. The thickness of 6 inches must be converted to feet (0.5 feet) before calculating. This gives us 1,200 ÷ 27 = 44.4 cubic yards of concrete needed.
Why the Other Options Are Wrong
Option B: 120 cubic yards
This answer results from forgetting to convert the 6-inch thickness to feet, incorrectly using 6 feet instead of 0.5 feet in the calculation, then dividing by 27.
Option C: 1,200 cubic yards
This is the volume in cubic feet (1,200), but the question asks for cubic yards. The student forgot to divide by 27 to convert from cubic feet to cubic yards.
Option D: 1,440 cubic yards
This answer comes from using 6 feet instead of 0.5 feet for thickness AND forgetting to convert from cubic feet to cubic yards (40 × 60 × 6 = 14,400 cubic feet, but shown as 1,440).
Memory Technique
Remember '27 to fly' - there are 27 cubic feet in a cubic yard (3×3×3=27). Always convert inches to feet first, then calculate volume, then divide by 27 for cubic yards.
Reference Hint
Look up concrete volume calculations and unit conversions in the construction materials chapter, specifically the section on concrete quantities and measurements.
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