How do I calculate the depreciation on my office equipment?

I have completed my 2016 tax return via TaxTim. On the depreciation of office equipment I indicated that I am depreciating my laptop which was bought for R20 000. The amount I came up with was R3 333. 33. SARS requires me to indicate to them how I got to the figure. Can you please assist so I can explain myself to SARS?

Kindly use our Wear and Tear calculator and save it as a pdf prior to uploading the document to SARS.