Why the CA at 30/6/17 is $414m rather than $455 as indicated in 30 June 2018?

On 1 April 2015 the construction of a fixed oil platform is completed and ready for use at a total cost of $500 million. The useful life of the rig is linked to the 25-year exploration rights granted to the company. Due to the specific nature of the platform it is deemed to have no realisable value (other than minimal scrap value) at any stage throughout its life. All impairment tests are therefore based on value-in-use estimations. On 30 June 2017 a rapid and significant decline in worid oil prices has provided an indication that the asset may be impaired. On this date, the rig's value in use is estimated to be $414 million. On 30 June 2018 a major contract was cancelled after one of the company's customers was declared bankrupt. This led directors to believe the value in use of the rig was now $374 million. Required Prepare the necessary journal entries to record adjustments for impairment on 30 June 2017 and 30 June 2018. 30 June 2017 Impairment loss- Oil rig Dr 41,000,000 Cr Acc. depreciation & impairment loss - Oil rig 41,000,000 Workings: Cost of $500m with 25 year life Dep. of $20m p.a. Acc. dep. after 24 years $45m. CA-$455m, RA -$414m so Impairment of $41m] 30 June 2018 Impairment loss Oil rig Dr 21,802,198 Cr Acc. depreciation& impairment loss - Oil rig 21,802,198 Workings: CA at 30I6/17 of $414m with 22% year life, Depreciation for year of $18,197,802 and CA of $395,802,198 RA of S374m so Impairment of $21,802,198)