Depreciation expense

Hello,

In Wiley book 3 reading of long lived asset example 2-2

Why divided the cost of new component although he said that calculate the depreciation in 2 scenario , in case using component method , and in case not using

I understand in using , But why calculated it in case not using that method

I mean why he calculated the depreciation of the new peace replaced at end of year 2

Please copy paste the exact problem text.