What is X?
24 81 63 26 412 8 25 X
6
The series is spaced incorrectly. When the spacing is correct it becomes: 2 4 8 16 32 64 128 256, which is an obvious doubling-up series.

6
The series is spaced incorrectly. When the spacing is correct it becomes: 2 4 8 16 32 64 128 256, which is an obvious doubling-up series.