### First-year decline in value deduction

**(a) Prime cost method**  
\[
\$30{,}000 \times \frac{1}{5} = \$6{,}000
\]  
**Deduction: $6,000**

**(b) Diminishing value method**  
\[
\$30{,}000 \times \frac{2}{5} = \$12{,}000
\]  
**Deduction: $12,000**

### Controlling authority
**Division 40 of the Income Tax Assessment Act 1997 (Cth)** (the decline in value / capital allowance rules).