Tangible Non-current assets

Dear Sir,
Could you please explain to me the answer to this question?

Tibet acquired a new office building on 1 October 2014. Its initial carrying amount consisted of:

$’000
Land 2,000
Building structure 10,000
Air conditioning system 4,000
Total 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 2015?

$’000

A
15,625

B
15,250

C
15,585

D
15,600

The aswer is A:
Six months’ depreciation is required on the building structure and air conditioning system.

$’000
Land (not depreciated) 2,000
Building structure (10,000 – (10,000/25 x 6/12)) 9,800
Air conditioning system (4,000 – (3,500/10 x 6/12)) 3,825
Total 15,625

So why is the depreciation of Air conditioning is calculated based on the cost of 3500, not 4000?