Fixed capital cost,depreciation multiplier and the rate of interest

Suppose in an economy with zero interest rate five new trucks are bought and afterwards the fleet is renewed by investing the annual depreciation quotas. The service life of a truck is four years. What will be the stationary size of the fleet? The assumed answer is four trucks. The correct answer is eight trucks. The difference is measured by a coefficient called depreciation multiplier. The value of the depreciation multiplier is examined for three typical time profiles of fixed assets, and its limits are found to be 1 ?μ?2. It is then shown how the rate of interest can be interpreted as a rate of growth in an economy with unchanged technology. If the rate of interest is made equal to the rate of growth of the economy, the ratio of discounted values of brand new fixed assets and fixed assets of balanced age-distribution is equal to the ratio between gross and net capital in the growing economy with no interest rate.