disposal of copyright

31 March 2021, Jessica sold a copyright for £28,800. The copyright had been purchased
on 1 April 2015 for £21,000 when it had an unexpired life of 15 years.
What is Jessica’s chargeable gain in respect of the disposal of the copyright
A £0
B £20,400
C £16,200
D £7,800
the solution in the text
ale proceeds 28,800
Less: Allowable element of acquisition cost (W) (12,600)
–––––––
Chargeable gain 16,200
–––––––
Working: Allowable element of acquisition cost
Remaining life at disposal = 9 years
Estimated useful life = 15 years

Allowable cost = £21,000 × 9/15 = £12,600
Hello Sir may you please help me understand why they used the remaining years 9 instead of the period of ownership 6