Financial reporting past question .

An entity purchases an aircraft that has an expected useful life of 20 years with no residual value The aircraft requires a substantial overhaul at the end of 5, 10 and 15. The aircraft cost $25 million and $5million of this figure is estimated to be attributable to the economic benefit that are restored by the overhauls. In year 6, the cost of the overhaul is estimated to be $6million. Calculate the annual depreciation charge for the years 1-5 and 6-10. Please help me to understand this question.