Hello, I am study for an exam coming up and i am desperatly trying to understand how the Average Book Value is $72,000 without know with Straight-Line Depr/salvage value

I may have missed it in my notes from class.

Computing AAR for the Project . . . . Sample Project Data: Year 0: CF = -165,000. Year 1: CF = 63,120 NI = 13,620. Year 2: CF = 70,800 NI = 3,300. Year 3: CF = 91,080 NI = 29,100. Average book value = $72,000. Required average accounting return = 25%. Average Net Income: ($13,620 + 3,300 + 29,100)/3 = $15,340. AAR = $15,340 / 72,000 = .213 = 21.3%. Do we accept or reject the project? 8-21