Final answer:
The question also includes an unrelated task about organizing conference duration data and the popularity of conference lengths, which would be assessed using statistical measures such as mean, median, or mode.
Step-by-step explanation:
The student's question is regarding the calculation of project duration for a stadium construction project. To determine the project duration, we would need more specific information related to the tasks involved and their respective durations. Given that we do not have this information, it is not possible to accurately calculate the project duration from the options provided (20, 22, 24, 23, 21 days).
For the secondary questions about organizing data for engineering conferences and choosing the length of a conference, we would create a chart to organize the conference durations and then use mean, median, or mode to decide on the optimal conference length based on the data (which seems to indicate that conferences are popularly lasting between three to five days).
Two reasons why three to five days might be popular for engineering conferences could be that this duration is sufficient for covering a comprehensive agenda without being too long to result in attendee fatigue, and it is a practical time frame that fits well into professionals' work schedules.