Question on PPE

Dear Chris, I have a question on PPE from the Kaplan Kit. Question: Tibet acquired a new office building on 1 October 20X4. Its initial carrying amount consisted of: $000 Land 2,000 Building structure 10,000 Air conditioning system 4,000 ––––––– 16,000 ––––––– The estimated lives of the building structure and air conditioning system are 25 years and 10 years respectively. When the air conditioning system is due for replacement, it is estimated that the old system will be dismantled and sold for $500,000. Depreciation is time?apportioned where appropriate. At what amount will the office building be shown in Tibet’s statement of financial position as at 31 March 20X5? Answer: $000 Land (not depreciated) 2,000 Building structure (10,000 – (10,000/25 × 6/12)) 9,800 Air conditioning system (4,000 – (3,500/10 × 6/12)) 3,825 –––––– 15,625 –––––– What I do not comprehend is the steps regarding the carrying value of the air con. I understand that the $3500 comes from $4,000 - $500, but I don't understand why is it being used to calculate the depreciation. Isn't $3,500 supposed to be the loss of the dismantling of aircon? Thanks for your help.