Cost Price (N) = 20000
Salvage value (N) = 5000
Annual depreciation (N) = 1000
Total depreciation = 15000
The formula to calculate the useful life is:
Useful life = (Cost Price - Salvage value) / Annual depreciation
Now, let's plug in the values:
Useful life = (20000 - 5000) / 1000
Useful life = 15000 / 1000
Useful life = 15 years
Therefore, the useful life represented by Y in the table is 15 years.