The Eimac tubes in a linear amplifier for radio transmission are estimated to provide 18,000 hours of operation before requiring replacement. A pair of tubes cost $20,000 and has no salvage value. Their use, expressed in hours over a 4-year period, is (1) 2,5000, (2) 4,500, (3) 3,000, (4) 3,900. Determine the current book value based upon the operating hour method of depreciation.