Depreciation of assets follow 20% the first year, and 10% every year thereafter. At what year should we stop buying the locks?

A B D E F G H I J K L M N O Q R S year1 year2 year3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 asset $ 2,000.00 $ 2,000.00 $ 1,600.00 $ 1,440.00 $ 1,296.00 $ 1,166.40 $ 1,049.76 $ 944.78 $ 850.31 $ 765.28 $ 688.75 $ 619.87 $ 557.89 $ 502.10 $ 451.89 $ 406.70 $ 366.03 $ 329.43 # assets 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 locks $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 sle $ 2,000.00 $ 2,000.00 $ 1,600.00 $ 1,440.00 $ 1,296.00 $ 1,166.40 $ 1,049.76 $ 944.78 $ 850.31 $ 765.28 $ 688.75 $ 619.87 $ 557.89 $ 502.10 $ 451.89 $ 406.70 $ 366.03 $ 329.43 aro before 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 1 1 aro after 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ale before $ 8,000.00 $ 8,000.00 $ 6,400.00 $ 5,760.00 $ 5,184.00 $ 4,665.60 $ 4,199.04 $ 3,779.14 $ 3,401.22 $ 3,061.10 $ 2,754.99 $ 2,479.49 $ 2,231.54 $ 2,008.39 $ 1,807.55 $ 1,626.79 $ 1,464.11 $ 1,317.70 ale after $ 2,000.00 $ 2,000.00 $ 1,600.00 $ 1,440.00 $ 1,296.00 $ 1,166.40 $ 1,049.76 $ 944.78 $ 850.31 $ 765.28 $ 688.75 $ 619.87 $ 557.89 $ 502.10 $ 451.89 $ 406.70 $ 366.03 $ 329.43 safeguard val $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 1 savings $ 6,000.00 $ 6,000.00 $ 4,800.00 $ 4,320.00 $ 3,888.00 $ 3,499.20 $ 3,149.28 $ 2,834.35 $ 2,550.92 $ 2,295.83 $ 2,066.24 $ 1,859.62 $ 1,673.66 $ 1,506.29 $ 1,355.66 $ 1,220.10 $ 1,098.09 $ 988.28 2 realized savings $ 5,000.00 $ 5,000.00 $ 3,800.00 $ 3,320.00 $ 2,888.00 $ 2,499.20 $ 2,149.28 $ 1,834.35 $ 1,550.92 $ 1,295.83 $ 1,066.24 $ 859.62 $ 673.66 $ 506.29 $ 355.66 $ 220.10 $ 98.09 $ (11.72) A B D E F G H I J K L M N O Q R S year1 year2 year3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 asset $ 2,000.00 $ 2,000.00 $ 1,600.00 $ 1,440.00 $ 1,296.00 $ 1,166.40 $ 1,049.76 $ 944.78 $ 850.31 $ 765.28 $ 688.75 $ 619.87 $ 557.89 $ 502.10 $ 451.89 $ 406.70 $ 366.03 $ 329.43 # assets 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 locks $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 $ 10.00 sle $ 2,000.00 $ 2,000.00 $ 1,600.00 $ 1,440.00 $ 1,296.00 $ 1,166.40 $ 1,049.76 $ 944.78 $ 850.31 $ 765.28 $ 688.75 $ 619.87 $ 557.89 $ 502.10 $ 451.89 $ 406.70 $ 366.03 $ 329.43 aro before 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 1 1 aro after 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ale before $ 8,000.00 $ 8,000.00 $ 6,400.00 $ 5,760.00 $ 5,184.00 $ 4,665.60 $ 4,199.04 $ 3,779.14 $ 3,401.22 $ 3,061.10 $ 2,754.99 $ 2,479.49 $ 2,231.54 $ 2,008.39 $ 1,807.55 $ 1,626.79 $ 1,464.11 $ 1,317.70 ale after $ 2,000.00 $ 2,000.00 $ 1,600.00 $ 1,440.00 $ 1,296.00 $ 1,166.40 $ 1,049.76 $ 944.78 $ 850.31 $ 765.28 $ 688.75 $ 619.87 $ 557.89 $ 502.10 $ 451.89 $ 406.70 $ 366.03 $ 329.43 safeguard val $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 $ 1,000.00 1 savings $ 6,000.00 $ 6,000.00 $ 4,800.00 $ 4,320.00 $ 3,888.00 $ 3,499.20 $ 3,149.28 $ 2,834.35 $ 2,550.92 $ 2,295.83 $ 2,066.24 $ 1,859.62 $ 1,673.66 $ 1,506.29 $ 1,355.66 $ 1,220.10 $ 1,098.09 $ 988.28 2 realized savings $ 5,000.00 $ 5,000.00 $ 3,800.00 $ 3,320.00 $ 2,888.00 $ 2,499.20 $ 2,149.28 $ 1,834.35 $ 1,550.92 $ 1,295.83 $ 1,066.24 $ 859.62 $ 673.66 $ 506.29 $ 355.66 $ 220.10 $ 98.09 $ (11.72)