The undepreciated capital cost balances at January $1,2023$ were as follows:

$-$ Class $13$ $$37,500$

Improvements on its leased head office premises which were rented in $2021$ for four years with two successive options to renew for five years and five years. Improvements had originally been made in $2021$ in the amount of $$45,000.$ Additional improvements were made in $2023$ at a cost of $$28,000.$

$2021$: $45000/(4+5)=5000$

$2023$: $28000/(2+5)=4000$ x $1\mathrm{.}5=6000$

Why do we multiply by x $1\mathrm{.}5$ in $2023?$ Explain clearly.