18In the case of any fixed asset which has a limited useful economic life, the amount of—
(a)its purchase price or production cost; or
(b)where it is estimated that any such asset will have a residual value at the end of the period of its useful economic life, its purchase price or production cost less that estimated residual value;
shall be reduced by provisions for depreciation calculated to write off that amount systematically over the period of the asset’s useful economic life.