Net book value change requires additions and depreciation details

Steps:

• Calculate NBV of sold vehicle: $30,000 cost -$21,000 accumulated depreciation = $9,000.
• Opening NBV (2017): $28,000; closing NBV (2018):$24,000; net decrease: $4,000.
• Formula: Closing NBV = Opening NBV + Additions - NBV disposed - Depreciation.
• Substitute: $24,000 =$28,000 + Additions - $9,000 - Depreciation → Depreciation = Additions -$5,000.
• Additions not provided, so depreciation cannot be determined.

Not enough information.

Final answer: Not enough information.