(a) Prime cost method:  
`$30,000 × (1 / 5) = $6,000`

**First-year deduction: $6,000**

(b) Diminishing value method:  
`$30,000 × (2 / 5) = $12,000`

**First-year deduction: $12,000**

Controlling authority: **Division 40 of the Income Tax Assessment Act 1997 (Cth)**, the capital allowance rules for decline in value of depreciating assets.