You may know this but I didn't. I know what straight line depreciation is and my reading tells me that double decling is 2 x Straight line. BUT this is not so! (I have verified this from more than one source including Cornelius Fitchner).

Look below - Straight Line is $9,000 annually in this case and so I assumed that DDL would be $18,000 - but look below and notice the two columns compared. Read the definition closely.

Good Luck.