The sandbox is 3' long, 3' wide so the sq footage of the sandbox is 3*3 = <<3*3=9>>9 sq ft
He needs 9 sq ft of sand to fill the sandbox and he can buy it in 3 sq ft bags so he needs 9/3 = <<9/3=3>>3 bags of sand
The sand costs $4.00 a bag and he needs 3 bags so it will cost him 4*3 = $<<4*3=12.00>>12.00 in sand
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