Difference on depreciation

Hello Sir, How are you, with regard to calculation of the old depreciation before the revaluation we should always take the cost over the life of the asset but why in this question they took the historic cost over the remaining life ,is it okay. Thanks, ------------------------------------------------------------------------------------------------------------------------------ Question The following trial balance extract relates to Topsy Co as at 30 April 20X6: $'000 $'000 Land at cost 800 Building: Valuation at 1 May 20X2 1,500 Accumulated depreciation at 30 April 20X5 90 Revaluation surplus at 30 April 20X5 705 On 1 May 20X2, when the carrying amount of the building was $750,000, it was revalued for the first time to $1.5m and its remaining useful life at that date was estimated to be 50 years. Topsy Co has correctly accounted for this revaluation in the above trial balance. However, Topsy Co has not yet charged depreciation for the year ended 30 April 20X6 or transferred the excess depreciation from the revaluation surplus to retained earnings at 30 April 20X6. In February 20X6, the land, but not the building, was independently valued at $950,000. This adjustment has yet to be made for the year ended 30 April 20X6. What is the balance on the revaluation surplus of Topsy Co as at 30 April 20X6 after the required adjustments have been made? The Answer $840,000 $ $ At 30 April 20X5 705,000 Increase in value of land in the year ($900,000 – $750,000) 150,000 855,000 Annual transfer to retained earnings Depreciation based on revalued amount ($1,500,000/50 years) 30,000 Depreciation based on historic cost ($750,000/50 years) (15,000) (15,000) At 30 April 20X6 840,000