I want to share a series of questions that are created by myself.
I will give a hint in 24 hours and my answer in 3 days if nobody could answer my question.
Previous ones are here:
Number Sequence Series-Question 1
Number Sequence Series-Question 2
Number Sequence Series-Question 3
Here is the new number sequence:
21870,1215,?,15
If you guys want some extremely challenging questions. Please check these two questions posted by me.