BPP Learning - Question

Chapter - Accounting for associates - Question page 183 Consolidated statement of financial position is to be reported at 31.12.20W6 Adjustment(answer): Freehold property John Co =1950 Paul Co =1250 Fair value adjustment =400 Additional depreciation (400*50/40)*6years(20W0-20W6) =(30) Total =3570 I don't understand why the additional depreciation should be divided by 40 years than 50 years as is ? Thank you