Number and Letter Series Cheat Sheet
2 Nov 2026
Quick answer
A quick reasoning revision sheet for number and letter series — the common patterns of difference, product, squares and cubes, alternating and mixed series, plus position-based letter series — with the step-by-step method for finding the next term and worked examples, for fast SSC, Banking and Railway revision.
In a series question you are given a sequence and must supply the missing or next term by spotting the rule that links each term to the next.
The common number patterns
| Pattern | Rule | Example |
|---|---|---|
| Constant difference | add/subtract a fixed number | 5, 9, 13, 17 (+4) |
| Growing difference | the gap itself grows | 2, 6, 12, 20, 30 (gaps 4,6,8,10) |
| Product / ratio | multiply by a fixed number | 3, 6, 12, 24 (×2) |
| Squares / cubes | n² or n³ | 1, 4, 9, 16, 25; 1, 8, 27, 64 |
| Alternating | two patterns interleaved | 1, 2, 4, 5, 7, 8 (+1, +2 alternately) |
| Mixed | combine ×/+ , e.g. ×2+1 | 2, 5, 11, 23 (×2+1) |
The method
- Find the differences between consecutive terms first — a constant or simple growing difference is the commonest answer.
- If the differences are not simple, test ratios (is each term a multiple of the one before?).
- If neither, check squares, cubes or an alternating pattern (look at every other term).
- Confirm the rule across the whole series before filling the blank.
Worked examples
2, 6, 12, 20, 30, ? — differences are 4, 6, 8, 10, so the next gap is 12: 30 + 12 = 42.
3, 6, 12, 24, ? — each term is ×2 of the one before: 24 × 2 = 48.
1, 4, 9, 16, 25, ? — these are 1², 2², 3², 4², 5², so next is 6² = 36.
Letter series
Letter series use the alphabet positions (A=1 … Z=26). Convert the letters to numbers, find the pattern, convert back.
C, F, I, L, ? — positions 3, 6, 9, 12 (each +3), so the next is 15 = O.
Quick tips
- Write the differences under the series — most number-series answers fall out of the difference row.
- For a letter series, always work in position numbers; never guess by "feel".
Revise the full method in the Number and Letter Series lesson, then practise series questions or browse more reasoning revision sheets.
How this was made: Drafted with AI assistance; every formula and worked figure recomputed independently; every fact checked against the source it cites.