Logical number sequence

[Toyotomi_cz]

There is a logical rule

 

[Steven G]

Yes it is.
Do you have a question about it?

 

[Toyotomi_cz]

What could be a question mark?

 

[ksdhart2]

Anything you want. Problems of this sort are mind reading exercises rather than math problems. The "right" answer is whatever the person who wrote the problem is thinking of, but there's infinitely many other possible answers. For instance, maybe ? = 2. Then there'd be a nice bijection to be made:

In	Out
0	7
1	4
2 ['?']	8
3	2
4	6
5	5
6	1
7	9

By a similar logic, the '?' could be 8 or 9 and also create a bijection. Or maybe it's something else entirely? Who really knows?