Q4 from Chapter 6-Depreciation

Sir, I am struggling with the following question and I hope you can help me.
Here’s question 4 from the test of chapter 6:
On 1 January 2000 Krin Co. brought a machine for $70,000.It was estimated that the machine’s useful life would be 7 years and its residual value $7,000.Two years later the useful life was revised to three remaining years and at 31 December 2003 the machine was sold fro $30,000.What is the profit on disposal?
My calculation is :
($70,000-$7,000)*2/7+($70,000-$7,000)*2/3+$30,000-$70,000=$20,000

but in the answer, the calculation would be:
($70,000-$7,000)*2/7+($70,000-$1,800-$7,000)*2/3+$30,000-$70,000=$8,000

So my question is, why we use the value of $52000 after 2 years, to calculate the depreciation charge instead of original cost$70000? I think It’s only the useful life is changing not the value.
I am a little confused about the figure I need to put on calculation to get depreciation charge whenever there’s a change in value(that is revaluation) and useful life.
Please help me with this.
Thank you,sir.