Purple Insurance Agency currently leases a van and pays $1.25 per mile for the annual mileage of 20,000. Now the agency considers using its own car that incurs the followings. Car purchase at $50,000 with predicted salvage of $10,000 after 4 years Operating and maintenance expenses at $2,500 per year. MARR=10% compounded annually . . Compute the unit cost per mile when using its own car, and choose the more economical option (i.e., lease vs. buy). [Show all factor notations involved in the calculation.]