Question 1176774
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From the post, the depreciation rate is  {{{20000/5}}} = 4000 monetary units per year.



THEREFORE, the depreciation function is


    y = 20000 - 4000*x,


where x is the time in years after the starting moment.



In 3 years,  y = 20000 - 4000*3 = 8000.
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Answered and solved.