Straight line depreciation

Hello sir..!

While calculating depreciation using straight line method ,if a percentage rate is given (say 20%), do we need to apply the percentage on cost or cost less residual value.

My understanding is that we have to apply the percentage on cost and not on cost less residual value.

The question which caused this doubt is FA Kaplan kit 2023-2024 question number 200.

It goes as follows:

A business has an accounting year end of 30 June. It purchased an item of plant on 1 April 20X5 as follows :

Cost $15000
3 year maintenance agreement $450

Total $15450

At the date of purchase the item of plant had an estimated useful life to the business of five years and an estimated residual value of $2000. This item of plant was traded in for a replacement item on 30 September 20X8 at an agreed valuaton of $5000.

It has been depreciated at 20% per annum on a straight line basis with a pro rated charge in the year of acquisition and disposal.

Calculate the profit or loss on disposal of the item of plant.

In the provided answer, they have calculated accumulated depreciation as follows:

(15000 cost -2000 residual value )*20%*3.5 years

Which equals to $9100

Shouldn’t it be 15000 cost * 20% * 3.5 years

Thanks in advance …