Process control equipment purchased for $78,000 by Debco, Incorporated generated a CFBT of $26,080 during the first year of its 10-year estimated life. This would represent a return of 31.2% per year if maintained throughout the 10 years. However, the corporate finance officer determined that the CFAT was only $18,000 for the first year, and it is expected to decrease by $1000 per year thereafter. If the president wants to realize an after-tax return of 12% per year, for how many years must the equipment remain in service?