YTAirlines includes the following statement about depreciation on its annual report:

We record property and equipment at cost and depreciate or amortize these assets on a straight line basis to their estimated residual values over their estimated useful lives. The estimated useful life for leasehold improvements is the shorter of lease term or estimated useful life. Depreciation and amortization expense related to our property and equipment was 2.3 billion dollars. Residual values for owned aircraft, engines, spare parts and simulators are generally 5% to 10% of cost.

The following information on their long term assets:

Dec 31, 2015

(in millions)

PPE, at cost: 44,158

Less: Accumulated depreciations & amortization: (15,823)

PPE, net : 28,335

a.) Ignoring the salvage value, what is the average expected useful life of YT's asset? And what is the average

age?

b.) If we use the 5% residual value, what is the useful life? What is useful if we use 10% residual value?

c) YT's sale revenue was $44,438 in 2015. If YT's internal rate of return is 20%, what is the annual benefit to YT's from its long term assets as a fraction of their revenue, if we ignore the residual value? And how does the answer change if the residual value is 5% of the cost of their long term asset?

d.) As a fraction of sales revenue, how much was the economic depreciation of their long term asset in 2016, if we ignore the residual value? and how would the answer change if the residual value is 10% of the cost of their long term asset?