how does the "55" come about?

7. On January 1, 2024, Landon Excavation Company purchased a new bulldozer for 120.000. The equipment had an estimated useful life of ten years and an estimated residual value of S10,000. On January 1, 2026, Landon determined that the life of the bulldozer should be revised from ten years to seven years. Assume sum-of-the-years- digits depreciation. REQUIRED: Calculate depreciation expense on the bulldozer for 2024, 2025, and 2026. 2024... Depreciation Expense Sum of years digits: Cost -residual value x Life to SY Doors X Depreciation Expenre 2024 120,000 - 10.000 10 i 55