NCA-Depreciation

Hello, I was hoping you could help me with the following question from the Kaplan exam kit:\ On 1 January 20x8, Wootton Co had a building in its books which cost $500,000, with a carrying amount of $405,000. On 1 July 20x8, the asset was valued at $600,000 and Wootton Co wishes to include that valuation in its books. Wootton Co's accounting policy is to depreciate buildings at the rate of 2% on a straight line basis. what was the depreciation charge included in the SOPL for the year ended 31 December 20x8? Answer: $12,500 --------------------------------------------------------------------------------------------------------------- Solution: Depreciation of $5000 for the first 6months (500,000 x 2% x 6/12) Accumulated Depreciation at the date of revaluation: 95000+5000=100,000 (10 years useful life passed) Total useful life of 50 years, with remaining useful life of 40 years. Depreciation for the 2nd half of the year: 600,000/40 x 6/12 = 7500 Total depreciation for the year: 7500+5000=12500 ----------------------------------------------------------------------------------------------------------------------- Sir, my question is how did they arrive at the 50 years of useful life? I can get behind deriving the 10 years of life passed. But I see no information in the question that would indicate the 50 years of useful life. Is there a method to derive it that I'm not aware of? also, I've copied the question as is and the solution. would really appreciate your help with this! Thanks!