Reasoning · Analytical puzzles · Topic 1 of 14
Number Series
Think of it like…
Your gullak (piggy bank) savings
Suppose you drop coins into a clay gullak every evening and write the total in a notebook: ₹3, ₹8, ₹15, ₹24, ₹35. Looking only at the totals, the pattern is not obvious. But ask “how much did I add each day?” and you see ₹5, ₹7, ₹9, ₹11 — every day you add ₹2 more than the day before. So tomorrow you will add ₹13 and the jar will hold ₹48. On another month, Aai doubles whatever is in your jar every Sunday; then the totals grow by multiplying, not adding. And if you and your sister fill two jars on alternate days, the notebook mixes two separate lists. A number series question is exactly this notebook: find what was added (or multiplied) each step, and the next total writes itself.
| In everyday life | In the exam topic |
|---|---|
| Total in the jar each evening | Term of the series |
| Money added each day | First difference (next term − this term) |
| “I add ₹2 more than yesterday” | Constant second difference |
| Aai doubling the savings every Sunday | Multiplication / ratio pattern (×2, ×2 + 1 …) |
| You and your sister writing on alternate days | Alternating series: two series mixed together |
The concept
A number series is a list of numbers made by a hidden rule. Your job is to find that rule and use it for the next term (or to catch the one term that breaks it). Always start by writing the differences between neighbouring terms below the series. If the differences are equal, it is an arithmetic series. If the differences themselves change by a fixed amount, look at the second differences.
If the differences grow very fast (roughly doubling or tripling), test ratios instead: ×2, ×3, or mixed rules like ×2 + 1 or ×3 − 2. If the terms are close to familiar numbers, compare them with squares (1, 4, 9, 16, 25 …) and cubes (1, 8, 27, 64, 125 …): many series are n² ± 1, n³ + n, or n(n + 1).
If nothing fits, split the series into odd and even positions. Often two simple series are woven together (alternating series). For “find the wrong number” questions, find the rule from the majority of terms, then check each term against it one by one.
Key points & formulas
- Step order: differences → second differences → ratios → squares/cubes → alternate positions
- Equal differences: arithmetic series (a, a + d, a + 2d …)
- Differences 2, 4, 6, 8 … or 5, 7, 9, 11 …: second difference is constant
- Fast growth: test ×k, ×k + c, ×k − c (e.g. 5, 11, 23, 47 is ×2 + 1)
- Know squares to 30² and cubes to 15³ by heart
- n(n + 1): 2, 6, 12, 20, 30 …; n² − 1: 3, 8, 15, 24 …; n³ + n: 2, 10, 30, 68 …
- Long series (7–8 terms) with a zig-zag: check odd and even positions separately
- Wrong-number questions: the wrong term usually makes two consecutive differences look odd
Worked example
Find the next term: 3, 8, 15, 24, 35, ?
Show solution
- First differences: 8 − 3 = 5, 15 − 8 = 7, 24 − 15 = 9, 35 − 24 = 11.
- Differences 5, 7, 9, 11 rise by 2 each time, so the next difference is 13.
- Next term = 35 + 13 = 48.
- Check with squares: the terms are 2² − 1, 3² − 1, 4² − 1, 5² − 1, 6² − 1, so the next is 7² − 1 = 48. Both methods agree.
Answer: 48
Exam shortcut
Look at the last digits and the size jump first. If a term roughly doubles each time, go straight to ×2 ± something and skip the difference table. If the terms sit just above or below squares or cubes, test n² ± 1 or n³ ± n before anything else.
Common traps
- Stopping after checking only two terms. A rule must fit every gap in the series, not just the first one.
- Missing an alternating series: if the differences jump up and down (e.g. +3, +1, +6, +2), split the series into two.
Practice · tap an option to check
Q1. Find the next term: 5, 11, 23, 47, 95, ?
Answer: C. Each term is (previous × 2) + 1: 95 × 2 + 1 = 191.
Q2. Find the next term: 2, 10, 30, 68, 130, ?
Answer: D. The terms are n³ + n for n = 1 to 5 (1 + 1, 8 + 2, 27 + 3, 64 + 4, 125 + 5). Next: 6³ + 6 = 216 + 6 = 222.
Q3. Find the next term: 4, 7, 8, 14, 16, 21, 32, ?
Answer: A. Two series are mixed. Odd positions: 4, 8, 16, 32 (×2). Even positions: 7, 14, 21, ? (+7). The next term is in an even position: 21 + 7 = 28.
Q4. Find the wrong number in the series: 3, 5, 9, 17, 33, 66, 129
Answer: B. The rule is ×2 − 1: 3 → 5 → 9 → 17 → 33 → 65 → 129. The term 66 should be 65.