Years of ownership of copyright

Hello Jilly,

Why for the following question about chargeable gains the no of ownership years of the copy right is 9 years not 6 years ?

Thanks,

On 31 March 2024, Jessica sold a copyright for £28,800. The copyright had been purchased on 1 April 2018 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 correct answer is C.

28,800 -(21,000 × 9/15) = £16,200