An interleaved sequence mixes two or more streams. Adjacent numbers may have no simple direct relationship because they belong to different patterns. Separating positions can reveal the structure.
Split the positions, not the values
Take 2, 20, 5, 18, 8, 16, ?. The odd positions contain 2, 5, 8: they increase by three. The even positions contain 20, 18, 16: they decrease by two.
The missing term is in position seven, so it belongs to the odd-positioned stream. Its value is 11. If the next even-positioned term were also requested, it would be 14.
| Position group | Terms | Rule |
|---|---|---|
| 1, 3, 5, 7 | 2, 5, 8, ? | Add 3 |
| 2, 4, 6, 8 | 20, 18, 16, ? | Subtract 2 |
Watch the terminology
Odd positions means the first, third and fifth places. It does not mean odd-valued numbers. The value 2 can belong to the odd-positioned stream because it appears first.
This distinction is especially useful when both streams contain a mixture of odd and even values.
Try a second example
For 3, 1, 6, 4, 12, 7, ?, separate 3, 6, 12 from 1, 4, 7. The first stream doubles, while the second adds three. Position seven continues the doubling stream, giving 24.
When to test this hypothesis
Try interleaving when neighboring differences or ratios look erratic but every other term seems organized. Do not use it as an automatic answer: check that each separated stream has enough evidence to support its rule.
After solving, reassemble the streams in their original order. This final check catches the common mistake of finding both rules correctly but reporting the next value from the wrong stream.