a St. Paul Lumber Company incurs a cost of $280 per hundred board feet in processing cer- tain "rough-cut lumber, which it sells for $320 per hundred board feet. An alternative is to produce a finished cut" at a total processing cost of $350 per hundred board feet, which can be sold for $415 per hundred board feet. What is the amount of (a) the differential revenue, (b) differential cost, and (c) differential income for processing rough-cut lumber into finished cut