Financial report context:
arconic and subsidiaries notes to the consolidated financial statements ( dollars in millions , except per-share amounts ) a . summary of significant accounting policies basis of presentation . the consolidated financial statements of arconic inc . and subsidiaries ( 201carconic 201d or the 201ccompany 201d ) are prepared in conformity with accounting principles generally accepted in the united states of america ( gaap ) and require management to make certain judgments , estimates , and assumptions . these may affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements . they also may affect the reported amounts of revenues and expenses during the reporting period . actual results could differ from those estimates upon subsequent resolution of identified matters . certain prior year amounts have been reclassified to conform to the current year presentation . the separation of alcoa inc . into two standalone , publicly-traded companies , arconic inc . ( the new name for alcoa inc. ) and alcoa corporation , became effective on november 1 , 2016 ( the 201cseparation transaction 201d ) . the financial results of alcoa corporation for all periods prior to the separation transaction have been retrospectively reflected in the statement of consolidated operations as discontinued operations and , as such , have been excluded from continuing operations and segment results for all periods presented prior to the separation transaction . the cash flows and comprehensive income related to alcoa corporation have not been segregated and are included in the statement of consolidated cash flows and statement of consolidated comprehensive ( loss ) income , respectively , for all periods presented . see note c for additional information related to the separation transaction and discontinued operations . principles of consolidation . the consolidated financial statements include the accounts of arconic and companies in which arconic has a controlling interest . intercompany transactions have been eliminated . investments in affiliates in which arconic cannot exercise significant influence are accounted for on the cost method . management also evaluates whether an arconic entity or interest is a variable interest entity and whether arconic is the primary beneficiary . consolidation is required if both of these criteria are met . arconic does not have any variable interest entities requiring consolidation . related party transactions . arconic buys products from and provides services to alcoa corporation following the separation at negotiated prices between the parties . these transactions were not material to the financial position or results of operations of arconic for all periods presented . effective may 2017 , upon disposition of the remaining common stock that arconic held in alcoa corporation , they are no longer deemed a related party . cash equivalents . cash equivalents are highly liquid investments purchased with an original maturity of three months or less . inventory valuation . inventories are carried at the lower of cost and net realizable value , with cost for approximately half of u.s . inventories determined under the last-in , first-out ( lifo ) method . the cost of other inventories is determined under a combination of the first-in , first-out ( fifo ) and average-cost methods . properties , plants , and equipment . properties , plants , and equipment are recorded at cost . depreciation is recorded principally on the straight-line method at rates based on the estimated useful lives of the assets . the following table details the weighted-average useful lives of structures and machinery and equipment by reporting segment ( numbers in years ) : .

Financial table:
segment: engineered products and solutions; structures: 29; machinery and equipment: 17
segment: global rolled products; structures: 31; machinery and equipment: 21
segment: transportation and construction solutions; structures: 27; machinery and equipment: 18

Additional report context:
gains or losses from the sale of assets are generally recorded in other income , net ( see policy below for assets classified as held for sale and discontinued operations ) . repairs and maintenance are charged to expense as incurred . interest related to the construction of qualifying assets is capitalized as part of the construction costs . properties , plants , and equipment are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets ( asset group ) may not be recoverable . recoverability of assets is determined by comparing the estimated undiscounted net cash flows of the operations related to the assets ( asset group ) to their carrying amount . an impairment loss would be recognized when the carrying amount of the assets ( asset group ) exceeds the estimated undiscounted net cash flows . the amount of the impairment loss to be recorded is calculated as the excess of the carrying value of the assets .

Question: what is the variation between the useful lives of the structures and machinery and equipment by the global rolled products segment?
Answer: 10