Dickerson Corporation had the following activities, traceable costs, and physical flow of driver units: Traceable Physical Flow of Activities Costs Driver Units Account inquiry (hours) $8,000 500 hours Account billing (lines) 8,000 200,000 lines Account verification (accounts) 3,000 2,000 accounts Correspondence (letters) 1,000 200 letters The above activities are used by departments A and B as follows: A B Account inquiry (hours) 50 hours 80 hours Account billing (lines) 25,000 lines 20,000 lines Account verification (accounts) 200 contracts 100 contracts Correspondence (letters) 20 letters 100 letters The cost per driver unit for the account billing activity is: