Please explain the exercise

Exercise #4

On July 1, 2014, Steff Inc. invested $720,000 in a mine estimated to have 800,000 tons of ore of uniform grade. During the last 6 months of 2014, 120,000 tons of ore were mined and sold.

Instructions Depletion costs per unit =

Assume that the 120,000 tons of ore were mined, but only 90,000 units were sold. How are the costs applicable to the 30,000 unsold units reported?

The balance sheet would show ore on hand a current asset at 30,000 tons times the cost per ton

2014 Balance sheet Income statement