**(a)** $6,000  
**(b)** $12,000  

**Working**  
Prime cost: \(30{,}000 \times (1/5) = 6{,}000\)  
Diminishing value: \(30{,}000 \times (2/5) = 12{,}000\)  

**Controlling authority:** Division 40 of the *Income Tax Assessment Act 1997* (ITAA 1997).