Machinery is purchased on July 1 of the current fiscal year for $180,000. It is expected to have a useful life of 4 years, or 20,000 operating hours, and a residual value of $20,000. Compute the depreciation for the last 6 months of the current fiscal year ending December 31 by each of the following methods: a) straight-line, b) declining-balance at twice the straight-line rate, c) units-of-production (used for 1,500 hours during the current year)