Buff Gym purchased exercise equipment at a cost of $107,000. In addition, Buff paid $3,000 for a special platform on which to stabilize the equipment for use. Freight costs of $1,600 to ship the equipment were borne by the seller. Buff will depreciate the equipment by the units-of-production method, based on an expected useful life of 55,000 hours of exercise. The estimated residual value of the equipment is $11,000. How many hours did Buff Gym use the machine if depreciation expense is $4,320?