6.45 Depreciation expense is an allocation of the cost of an asset over the asset's useful life. Entities assess the useful life of their various assets. The notes to the accounts for two airlines reveal the following variation in useful life estimates. Air New Zealand Ltd depreciates its aircraft eet on a straight-line basis to an estimated residual value over their economic lives as follows: Aircraft 13 years Residual values are reviewed arlnualh.r Engine overhauls Period to next overhaul Source: Air New Zealand Ltd 2015, annual financial results, p. 15. The Qantas Group depreciates its aircraft eet as follows: Aircraft and engines 2520 years 010 per cent residual vaiue Aircraft spare parts 15.20 years III'20 per cent residual value Source: Qantas Airways Ltd 2015, Annual report, p. 95. Required a. Comment on the effect of these differing depreciation rates on the profit of Air New Zealand Ltd and the Qantas Group. I). Can you suggest any reasons why the choice of useful lives would differ across the entities