### (a) Prime cost method

\[
\$30{,}000 \times \left(\frac{1}{5}\right)=\$6{,}000
\]

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

### (b) Diminishing value method

\[
\$30{,}000 \times \left(\frac{2}{5}\right)=\$12{,}000
\]

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

The controlling authority is **Division 40 of the Income Tax Assessment Act 1997 (Cth)**, particularly sections **40-70** (diminishing value) and **40-75** (prime cost).