Question 1
A machine needs to process 4 jobs. Processing times:
- Job D: 79 minutes
- Job E: 33 minutes
- Job B: 34 minutes
- Job A: 87 minutes
The machine breaks down at 70 minutes and takes 39 minutes to repair.
Jobs are scheduled using Shortest Processing Time (SPT) first rule.
What is the total completion time (makespan) after handling the breakdown?
Step-by-step solution (Breakdown Recovery):
1. Original SPT order: Job E โ Job B โ Job D โ Job A
2. Simulate processing with breakdown:
- Job E: 0 โ 33
- Job B: 33 โ 67
- Job D: Starts at 67, breakdown at 70 (3 min completed), repair 39 min, resume 76 min โ completes at 185
- Job A: 185 โ 272
3. Total makespan: 272 minutes
4. Delay caused by breakdown: 39 minutes
Answer: 272 minutes
Key Strategy: Simulate the timeline, account for breakdown during active job processing.
1. Original SPT order: Job E โ Job B โ Job D โ Job A
2. Simulate processing with breakdown:
- Job E: 0 โ 33
- Job B: 33 โ 67
- Job D: Starts at 67, breakdown at 70 (3 min completed), repair 39 min, resume 76 min โ completes at 185
- Job A: 185 โ 272
3. Total makespan: 272 minutes
4. Delay caused by breakdown: 39 minutes
Answer: 272 minutes
Key Strategy: Simulate the timeline, account for breakdown during active job processing.