Machine A costs $30,000 to purchase and is worth $9,000 in 5 years at the end of its service life. Machine B costs $15,000 to purchase and is worth $2,000 in 2 years at the end of its service life. Assume that these machines are needed for 20 years (required service period) and can be repurchased at the same price in the future. (use 13% annual interest rate). What is the best (MOST EFFICIENT) Time Period for an NPV or NFV analysis (years).