4. In the highlighted cells, indicate the useful life and salvage value percentage. Then calculate the annual depreciation expense that Delta and Singapore would record for each $100 of aircraft cost. For example, if salvage value was 4% of cost and the useful life was 4 years then the calculation would be (100 - (100*.04))/4 years = 24. Your answer is the percentage of aircraft cost depreciated each year (e.g., 24%), which allows you to compare firms with different $ amounts of aircraft.