AT Company purchased a tractor at a cost of $60,000. The tractor has an estimated salvage value of $10,000 and an estimated life of 8 years, or 12,000 hours of operation. The tractor was purchased on January 1, 2012 and was used 2,400 hours in 2012 and 2,100 hours in 2013. What method of depreciation will produce the maximum depreciation expense in 2013?