Non-controlling interest confusion

Dear Sir

I hope this message finds you well (apologies in advance for the length of the question, it’s how it was written )
I’m confused with the following question:

Donna Co acquired 80% of the equity share capital of Blitsen Co on 1 January 20X4 when the retained earnings of Blitsen Co were $40,000. The fair value of the non-controlling interest at this date was $25,000.
At 31 December 20X4, the equity capital of Blitsen Co was as follows:

Share capital 40000
Share premium 10000
Retained earnings 60000

During the year Blitsen Co sold goods to Donna Co for $20,000. This price included a mark-up of$12,000 for profit. At 31 December 20X4, 50% of these goods remained unsold in the inventory of Donna Co.
What is the value of the non-controlling interest in the Donna Group at 31 December 20X4, for the purpose of preparing the consolidated statement of financial position?

Their answer:
Fair value of NCI at date of acquisition 25,000
NCI share of retained post-acquisition earnings: 20% × $(60,000 – 40,000) 4,000
sum total: 29,000
NCI share in unrealised profit: 20% × 50% × $12,000 = (1,200)
Non-controlling interest at 31 December 20X4 = 27,800

Now, according to what I’m relatively confident on,(for this specific question) the Non controlling Interest is:

The Fair Value of NCI at Acq plus percentage ownership of post acq ret earnings less unrealised profit. to summarise:

25000 + 20%x20000 (60000 in yr – 40000@acq) – 50%x12000 = 25000 + 4000 – 6000= 23000
Why are the answers different?
Why is the NCI sharing the unrealised profit since it was the Sub that sold to the Parent, and the latter who still had goods in its inventory?

thanks in advance
regards

PS
I wanted to say thanks for all the great work you and your team do with this site and the help you offer. It’s truly useful!