\[Straight{\text{ }}line{\text{ D}}epreciation{\text{ }}expense = {\text{ }}\frac{{Cost - Residual{\text{ }}value}}{{Useful{\text{ }}life}}\]
\[DDB{\text{ }}depreciation = 2 \times \frac{{Cost - Accu\mulated{\text{ }}depreciation}}{{Useful{\text{ }}life}}\]
Where: DDB is the Double declining balance method