A number series always has exactly ONE rule connecting every term — the trick is testing the rule on at least two gaps before trusting it.
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An arithmetic series adds the same amount each time; a geometric series multiplies by the same amount each time.
📌 Spotting the series type
Arithmetic: same DIFFERENCE between terms. 5, 9, 13, 17 — each term adds 4. Next term: 21.
Geometric: same RATIO between terms. 3, 9, 27, 81 — each term multiplies by 3. Next term: 243.
Test the rule on at least TWO gaps before trusting it for the whole series.
When a series is not simply arithmetic or geometric, look at the DIFFERENCES between the terms — sometimes those differences form their own pattern.
📊 Worked example
| Series | 2 | 5 | 10 | 17 | 26 |
|---|---|---|---|---|---|
| Difference | — | 3 | 5 | 7 | 9 |
The differences themselves go up by 2 each time (3, 5, 7, 9, …). The next difference is 11, so the next term is 26+11 = 37.
A "wrong term" question hides one broken link in an otherwise perfect chain — find the rule from the terms that DO fit.
Method: check the pattern using the first three or four terms (skipping any suspect one), confirm it on a term you trust, then apply it to find which term breaks the rule.
12 questions on number series, difference patterns and finding wrong terms.
