I have a better suggestion for the mathematically challenged....since you all have computers, just type in "soil calculators" in google. From there you have a choice of free on-line calculators. Who has time for math these days?
Like tractors and the like? see below -
Kubota L39 w/grapple, pallet forks, BH & QA buckets, Thumb and more!
John Deere 2520, 210Cx, 46BH
Cub Cadet 3204 with Blower & Simms Cab,
Ford 2006 F550 turbo diesel 4x4 w/11' mason dump, Wright Stander RH 52",
Better Outdoor Product Quick 32" WB mower, and more !!