percentages.... I have a sheet of plywood, say 10 sq. ft. I sell two pieces. Then, Jim bought a 5 sq ft piece while Joe bought a 2 sq. ft piece. The rest of the sheet of plywood is no good to me, so I want to charge the remaining of plywood evenly to both customers for the percentage of their plywood usage. The remaining area is 3 sq. ft.. How much should Jim and Joe be charged for? I need to derive a formula for this but I am stumped...

If you want to charge in proportion to usage, then charge Jim $5/(5+2) = 5/7$ of the leftover plywood, and Joe $2/7$ of the leftover. That means Jim is charged for about $2.14$ sq ft of the plywood and Joe for the rest.