Atlas Gym purchased exercise equipment at a cost of $100,000. In addition, Atlas paid $2,000 for a special platform on which to stabilize the equipment for use. Freight costs of $1,200 to ship the equipment were borne by the seller. Atlas will depreciate the equipment by the units-of-production method, based on an expected useful life of 50,000 hours of exercise. The estimated residual value of the equipment is $10,000. How many hours did Atlas Gym use the machine if depreciation expense is $3,680?