meca concrete purchased a mixer on January 1st 2011 at a cost of $45,000. straight-line depreciation for 2011 and 2012 was based on an estimated 8-year life of $3,000 estimated residual value. in 2013 meca revised its estimate and now believes the mixer will have a total service life of only six years and that the residual value will be $2,000. compute depreciation for 2013 and 2014.