AAR=average NI/ average book value

how do i calculate average book value?

You're trying to determine whether to expand your business by building a new manufacturing plant. The plant has an installation cost of $12.7 million, which will be depreciated straight-line to zero over its four-year life. If the plant has projected net income of $1,924,300,$1,977,600,$1,946,000, and $1,399,500 over these four years, respectively, what is the project's average accounting return (AAR)? (Do not round intermediate calculations and enter your answer as a percent rounded to 2 decimal places, e.g., 32.16.)