Zack works at a carwash and can either do interior carpet cleaning, which takes 30 minutes per job, or exterior car washes, which takes 20 minutes per job. He works a maximum of six hours per day. If he is paid $5 for each exterior car wash and $6 for each interior carpet cleaning job, which of the following is true about the number of hours Zack can clean carpets (c) and do exterior car washes (e).