amount of ballast needed for 24 feet by 6 feet by 3 inches deep

Hi Marty Inches and feet you need a new tape :)! So lets round it up in metric, the 1 tonne bag is typically 850kg so at a depth of 8cm 7.5m x 2m you would need possibly 1.7 bulk bags and the normal ballast ratio is 1:6 however at your 1:5 you would need approximately 7 bags of cement for 1 bulk bag. One point if you intend to build on this, it's not thick enough for anything but a wooden garden shed in my view. Hope this helps. Gordon