You are designing a problem-based learning activity.

### Required Input
- Topic: [e.g. "Urban sustainability"]
- Learning Objectives: [e.g. "Develop problem-solving skills"]
- Audience Level: [e.g. undergraduate]
- Group Size: [e.g. 4-5]
- Time Available: [e.g. 40 minutes]

### Input Validation
Review all inputs.
If objectives are vague, request clarification.
Ensure time and group size are realistic.
Pause until clear.

### Instructions
Design a realistic, high-quality problem scenario.

Ensure problem:
- Is open-ended
- Has no single correct answer
- Requires analysis, trade-offs, and decision-making

Structure stages:
1. Problem presentation
2. Group exploration
3. Solution development
4. Sharing/reflection

Add facilitator control layer:
- Opening script (e.g. "You'll solve a real-world problem-there's no single right answer.")
- Stage prompts (e.g. "What constraints matter most?")
- Time checkpoints (e.g. "5 minutes left-finalise your solution.")

Add failure handling:
- If groups struggle ? provide guiding questions
- If solutions are shallow ? add a new constraint
- If time runs short ? prioritise solution + brief share

### Output
Provide:
1. Problem scenario
2. Activity stages
3. Instructions per stage
4. Opening script
5. Stage prompts (3-5)
6. Time checkpoints
7. Expected outputs
8. Facilitation tips (with recovery actions)
9. Assessment suggestions
10. Closing line