Answer:
Friendly Farm Company
Schedule of Straight-line, Units of Production, and Double Declining Balance:
                              Straight-line     Units of Production    Double Declining
Year 1 Book value   $350,000        $350,000                               $350,000
Depreciation Exp.     $80,000          $96,000 (30,000*$3.20)      $175,000
Year 2 Book value $270,000        $254,000                                 $175,000
Depreciation Exp.    $80,000           128,000 (40,000*$3.20)          87,500
Year 3 Book value $190,000         $126,000                                  $87,500
Depreciation Exp.   $80,000             64,000 (20,000*$3.20)          43,750
Year 4 Book value $110,000           $62,000                                  $43,750
Depreciation Exp.    80,000              32,000 (10,000*$3.20)         $13,750
Residual value       $30,000           $30,000                                  $30,000
Explanation:
a) Data and Calculations:
Cost of new machine = $350,000
Estimated useful life = 4 years or 100,000 hours
Residual value = $30,000
Usage of machine:
Year 1 = 30,000 hours
Year 2 = 40,000 hours
Year 3 = 20,000 hours
Year 4 = 10,000 hours
Units of Production = $320,000/100,000 = $3.20 per unit
Depreciable amount = $320,000 ($350,000 - $30,000)
Straight-line method, Depreciation per year = $80,000 ($320,000)
= 25% (100/4).
Depreciation expense, using Double-Declining Balance rate = 25% * 2 = 50%:
Year 1 = $350,000 * 50% = $175,000
Year 2 = $175,000 * 50% = $87,500
Year 3 = $87,500 * 50% = $43,750
Year 4 = $13,750 ($43,750 - $30,000)
b) These different methods still arrive at the same end result as shown above.  Note that depreciation is an accounting estimate which spreads the cost of an acquired long-term asset over its useful life.