Profit after replacement of asset

Sir, I have a doubt regarding this question (from Kaplan), please help me out.

—Question—
A business has an accounting year end of 30 June. It purchased an item of plant on 1 April 20X5 as follows:
Cost $15000
3 year maintenance agreement $450
Total $15450

At the date of purchase, the item of plant and equipment had an estimated useful life to the business of five years and an estimated residual value of $2,000. This item of plant was traded in for a replacement item on 30 September 20X8 at an agreed valuation of $5,000.
It has been depreciated at 20% per annum on a straight-line basis, with a pro-rated charge in the year of acquisition and disposal.
Calculate the profit or loss on disposal of the item of plant.
$ Profit / loss* * Delete which does not apply

—-What I did—

I did this- 15000 x 20% =3000 = depreciation per year,

Depreciation as on-

30 June 20X5- 750 (3 months APR to June)
30 June 20X6- 3k
30 June 20X7- 3k
30 June 20X8- 3k
30 Sep 20X8- 750 (3 months June to sep)

Accumulated depreciation= 10500
Carrying Amount= 15000-10500 = 4500

So profit as =5000 – 4500 = 500 (profit)

I am not understanding how the answer is 900 loss.
And how did they calculate 9100 as accumulated depreciation. Please tell me what I am doing wrong in the above calculation