Answer:
generally you need to determine the cost per unit, but in this case you are given a percentage of depletion = 10% x $225,000 = $22,500 which determines the inventory value (or depletion expense if the timber is sold) during the year.
the journal entry should be:
December 31, 20xx
Dr Timber inventory 22,500
Cr Accumulated depletion - timber tract 22,500