Apportionment of deductions where premises or assets are partly used for business limits allowable expenses to a fair proportion. Where premises are partly used as a dwelling and partly for business, deductions for rent, repairs and the other provision of section 30 are limited to the proportion attributable to business use as determined by the Assessing Officer. Where building, machinery, plant or furniture is not exclusively used for business, deductions for repairs, depreciation and related allowances are restricted to a fair proportion based on the asset's user for business, as determined by the Assessing Officer.