a) On January 1, the company retired a piece of machinery that was purchased on January 1, 2009 for $6 000. It had a useful life of six years and no residual value. b) On June 30, the company sold a computer purchased on January 1, 2010. It was sold for $600. The computer cost $4 000 and had a useful life of six years with a residual value of 5250. c) On January 1, the company discarded a delivery truck that was purchased on January 1, 2010. The truck cost $30 000. it was amortized based on a six-year useful life with a $3 000 residual value