why is average net investment divided by two and not over 3?

also, can you explain the weaknesses of ARR? thank you

(1) Accounting (Average) Rate of Return Method Investment opportunities A and B each cost $9,000 and have a useful life of 3 years, with no salvage value (assume S-L depreciation). Project A: Acctg. Profit $3,000 Project B: Acctg. Profit $1,000 $2,000 S3,000 Cash Flow $6,000 Year 1: Year 2: Cash Flow $4,000 $5,000 $6,000 Year 1: Year 2: S2,000 Year 3: S1,000 $5,000 Year 3: Avg.NI $2,000.44 for bo ARR Avg.Net Investment $4,500 Avg. Net Investment- (9,000+0)/2 4,500 Weaknesses of ARR? (1) AlHCFs? (2) TV of $? (3) Rik