A plumbing company outfits six panel vans with $5,000 of supplies each Saturday and sends them out into the field on service calls Monday through Friday where the parts are installed and billed to their customers. Its warehouse, which is used to restock the vans on Saturdays, holds $20,000 of inventory. The company bills customers an average of $20,000 every two weeks. How long does the company hold the average part? a. 1.5 weeks b. 1 week c. 2 weeks d. 2.5 weeks