Company has three trucks. One of the new trucks acquired will not depreciate this quarter. An old truck is fully depreciated. The third truck was acquired at a cost of $25000, has a salvage value of $7000, is depreciated on a miles driven basis(units of production depreciation), and has an estimated service life of 120,000 miles. The truck was driven 3,560 miles during the quarter. What is the depreciation expense amount. 25000-7000 = 12000 , divide this by 120,000 = 0.1 , multiply to 3560 = 356 , is this correct?

Your formula is correct, however your math is incorrect.

$25,000 cost - 7,000 salvage value = $18,000 depreciable base.

$18,000 depreciable base / 120,000 = $0.15 depreciation unit per mile

$0.15 depreciation unit per mile * 3,560 miles = $534 depreciation expense.