Explanation:
The computation of the ending inventory and the beginning inventory is shown below:
Ending inventory is as follows
December = 438,00 units × 5.5 gallons × 15% =
         = 36,135 units
January =  41,000× 5.5 gallons × 15%
       = 33,825 units
February = 50,250 units × 5.5 gallons × 15%
        = 41,457 units
And, the beginning inventory for January is December ending inventory i.e 36,135 units