810 slabs of concrete - Math Central

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Hi,
My name is Wesley . I have a volume question to check my math for my Eagle Scout project.

I'm making a 4" x 4" x 2" (Inches) concrete slab (approximately 750 of them, I'm going to plan for 810 for extra material). I'm trying to find out how many bags of cement I need. Each 60 lbs bag makes approximately .5 cu/ft.

Here's my work:

81 slabs = 9' X 9' X 2"= 81 sq ft X 1/6 = 13.5 cu ft

810 slabs = 13.5 cu ft X 10 = 135 cu ft

1 cubic yard = 135 cu ft / 27 = 5 cu/yd

1-60 lbs bag of cement = .5 cu ft
2-60 lbs bag of cement = 1 cu ft
6-60 lbs bag of cement = 1 cu yd

30-60 lbs bag of cement = 5 cu yds

Thank you
Wesley

Hi Wesley,

There is an error in your "81 slabs" calculation and also you don't need to use cubic yards at all.

81 slabs forms a square which is 9 slabs by 9 slabs. Each slab is 4" long so 9 slabs is 4 × 9 = 36 inches which is 3 feet. Hence 81 slabs forms a square that is 3 feet by 3 feet. Hence the 81 slab square which is 2" deep has a volume of 3' × 3' × 1/6' = 1.5 cubic feet.

How many cubic feet are in 810 slabs? Each bag of cement makes approximately half a cubic foot so how many bags are needed?

I hope this helps,
Harley

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