7-21. A firm can purchase a centrifugal separator (5-year MACRS property) for $20,000. The estimated salvage value is $3,000 after a useful life of six years. Operating and maintenance (O&M) costs for the first year are expected to be $2,000. These O&M costs are projected to increase by $1,000 per year each year thereafter. The income tax rate is 40% and the MARR is 15% after taxes. What must the uniform annual benefits be for the purchase of the centrifugal separator to be economical on an after-tax basis? (7.9)