First find the area of the lot by multiplying the width and the length: 120 ft * 60 ft = <<120*60=7200>>7200 sq ft.
Then find the area of the square covered in concrete: 40 ft * 40 ft = <<40*40=1600>>1600 sq ft.
Then subtract the concrete area from the total area to find the grassy area: 7200 sq ft - 1600 sq ft = <<7200-1600=5600>>5600 sq ft.
Finally, divide this area by the area each bag covers to find the number of bags: 5600 sq ft / 56 sq ft/bag = <<5600/56=100>>100 bags.
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