Starling co. is considering disposing of a machine with a book value of $12,500 and estimated remaining life of five years. the old machine can be sold for $1,500. a new high-speed machine can be purchased at a cost of $25,000. it will have a useful life of five years and no residual value. it is estimated that the annual variable manufacturing costs will be reduced from $26,000 to $23,500 if the new machine is purchased. the total net differential increase or decrease in cost for the new equipment for the entire five years is a. increase of $11,000 b. decrease of $15,000 c. decrease of $11,000 d. increase of $15,000