Answer:
a) $15 per ton
b)
year 1 depletion expense 225,000
year 2: 270,000
c) journal entries
depletion exense 225,000 debit
  coal mine deposit   225,000 credit
depletion exense 270,000 debit
  coal mine deposit   270,000 credit
Explanation:
$600,000 cost / 40,000 tons = $ 15 depletion per ton
now we multiply this rate by the amount extracter per year:
Year 1: 15,000 x 15 = 225,000
Year 2: Â Â 18,000 x 15 = 270,000
we charge the expense and deduct the mine reserve account,