Great american oilchange (gao) sells a combined oil change service and parts package for $30. a customer who supplies the parts (oil and filter) is charged $20 for only the service, whereas a customer who buys only the oil and filter for do-it-yourself use) is charged $20 for the parts. the parts cost gao $14. determine the dollar amount of revenue from the oil change service versus the sale of parts for each combined oil change package. tip: to calculate the percentage of the combined package that relates to the service, divide the stand-alone selling price of the service by the combined stand-alone selling prices of both the service and parts.