1. Problem: Two machines pack bricks at rates 24000 and 12000 bricks per hour. Find time to pack 12000 bricks together. 2. Formula: Total rate = sum of individual rates. Time = Total bricks / Total rate. 3. Calculate total rate: $$24000 + 12000 = 36000$$ bricks/hour. 4. Calculate time: $$\text{Time} = \frac{12000}{36000} = \frac{1}{3}$$ hours. 5. Convert to minutes: $$\frac{1}{3} \times 60 = 20$$ minutes. Answer: 20 minutes (option a).