I am totally new to Agile methodology and learning it. I have seen lots of tutorials and videoes and all very helpful. So, I have one unanswered question from my existing study. The situation is like

In beginning, there were total 100 story points. As part of Sprint 1, 10 are completed this 90 points are still remaining but 10 additional points are added.This makes again 100 story points.

In sprint 2, 20 story points completed but no further additional in any of the sprints so total points become 80.

Again in Sprint 3, 15 story points are completed and this story points come down to 65 points. and the cycle goes on. By burndown chart, we can estimate some values like:-

Team's average velocity is 15.

In 8 sprints total 120 story points will be covered.

To complete the work it will take around 7 sprints.

Unanswered question

4. Product Backlog changing rate (My answer is 65 but I am not sure)

If someone can answer this and explain the way to calculate that will be very helpful.

I am totally new to Agile methodology and learning it. I have seen lots of tutorials and videos and they've all been very helpful. However, I have one unanswered question from my existing study. The situation is:

In the beginning, there were a total of 100 story points. As part of Sprint 1, 10 were completed. 90 points were still remaining but 10 additional points were added. This makes, again, 100 story points in total.

In sprint 2, 20 story points were completed but none were added, so total points became 80.

In Sprint 3, 15 story points were completed. Total story points come down to 65. The cycle goes on. By the burndown chart, we can estimate some values like:-

Team's average velocity is 15.

In 8 sprints total, 120 story points will be covered.

To complete the work it will take around 7 sprints.

Unanswered question

4. Product Backlog changing rate (My answer is 65 but I am not sure)

If someone can answer this and explain the way to calculate that, it will be very helpful.