Divisional ROI

Pro is a division of Mo and is an investment centre. The head office controls finance, HR
and IT expenditure but all other decisions are devolved to the local centres.
The statement of financial position for Pro shows net value of all assets and liabilities to be $4,500m at the start of the year and $4,890m at the end. It carries no debt itself although the group has debt liabilities.
The management accounts for income read as follows:
$m
Revenue 3,500
Cost of sales 1,800
Local administration 250
IT costs 50
Distribution 80
Central administration 30
Interest charges 90
Net profit 1,200
Ignore taxation.
What is the divisional ROI to the nearest % point? (1 d.p.)
The answer should be 1370/4890=28% but it is 1370/4500=30.4%.
Don,t you think the end year net value should be used instead?Why the start year value was used?