RRB Group D Reasoning: Number Series and Coding-Decoding with Worked Examples
Solve RRB Group D number series and coding decoding questions faster with a four check method, worked examples, common traps and a weekly mock routine.
Number series and coding-decoding appear regularly in the General Intelligence and Reasoning section of the RRB Group D CBT. Both reward a fixed checking order more than raw speed. This guide gives you that order, shows it on worked examples, and ends with a practice routine you can run on RRB Group D mock tests.
Always confirm the current exam pattern and syllabus in the official notice on your regional RRB website (for example rrbcdg.gov.in) before planning.
Why a checking order matters
Most wrong answers in series questions come from testing patterns randomly. If you check in the same order every time, you either find the rule within 30–40 seconds or you know to skip.
The four-check method for number series
- First differences. Subtract each term from the next.
- Second differences or ratios. If differences are not constant, look at differences of differences, or divide terms.
- Alternating series. Split odd and even positions into two separate series.
- Squares, cubes and primes. Compare each term to n², n³, or nearby primes (±1, ±2).
Worked example 1
Series: 3, 7, 15, 31, 63, ?
- Differences: 4, 8, 16, 32 → doubling.
- Next difference = 64, so the answer is 127.
- Check: each term is also 2 × previous + 1 (63 × 2 + 1 = 127). Two rules agree, so you can mark confidently.
Worked example 2
Series: 2, 12, 4, 15, 6, 18, ?
- Differences jump up and down, so try check 3.
- Odd positions: 2, 4, 6 → next is 8.
- Even positions: 12, 15, 18.
- The missing term is in an odd position: 8.
Worked example 3
Series: 1, 9, 25, 49, 81, ?
- These are 1², 3², 5², 7², 9² → next is 11² = 121.
Worked example 4 (wrong-term question)
Find the wrong term: 5, 11, 23, 47, 96, 191
- Rule: × 2 + 1. 5 → 11 → 23 → 47 → 95 → 191.
- 96 is wrong; it should be 95.
Coding-decoding: three common types
| Type | What to check first | Example clue |
|---|---|---|
| Letter shift | Position values (A=1 … Z=26) | CAT → DBU (+1 each) |
| Reverse / mirror | Opposite letter pairs (A↔Z, B↔Y) | GO → TL |
| Number coding | Sum or product of letter positions | BAD = 2+1+4 = 7 |
Write the alphabet with positions once at the top of your rough sheet. It saves time on every coding question in the paper.
Worked example 5
If MANGO is coded as NBOHP, how is APPLE coded?
- M→N, A→B, N→O, G→H, O→P: each letter +1.
- APPLE → BQQMF.
Worked example 6
If in a code GOLD is written as TLOW, what is the code for SILK?
- G(7)↔T(20), O(15)↔L(12), L(12)↔O(15), D(4)↔W(23). Each pair adds up to 27, so it is the opposite letter.
- S↔H, I↔R, L↔O, K↔P → HROP.
Worked example 7
If CAB = 6 and DOG = 26, what is BAT?
- C+A+B = 3+1+2 = 6. D+O+G = 4+15+7 = 26.
- B+A+T = 2+1+20 = 23.
Common traps
- Stopping after one matching rule. Check the rule on every term, not just the first three.
- Counting the wrong position in alternating series. Write the position numbers above the terms.
- Mixing forward and opposite letter coding. Test one letter both ways before decoding the full word.
- Spending too long. If no rule appears after the four checks, mark the question for review and move on.
A weekly practice routine
| Day | Task |
|---|---|
| Mon | 20 number-series questions with a timer, using the four checks |
| Tue | 20 coding-decoding questions; write the alphabet sheet from memory first |
| Wed | Topic test in a RRB Group D sprint series |
| Thu | Review: list every wrong answer and which check would have caught it |
| Fri | Mixed reasoning set: series, coding, analogy, classification |
| Sat | One full-length RRB Group D CBT mock |
| Sun | Rest or a light revision of your error log |
Using your mock results
After each full mock, look at the reasoning section separately. Note the time spent per question and the questions you skipped. If series questions take more than a minute on average, go back to the four-check order. You can compare your scores across exams on Scholareum mock tests and plan around dates in the exam calendar.
Related guides
- RRB NTPC maths: time management and question selection
- RRB ALP basic science: units, formulas and numerical practice
- Browse all Scholareum test series
FAQs
How do I start a number series question? Find the first differences. If they are not constant, check second differences or ratios, then try splitting odd and even positions.
What is the fastest way to solve coding-decoding? Write letter positions (A=1 to Z=26) once, then test a simple shift and an opposite-letter rule on the first letter before decoding the whole word.
How many mocks should I take before the CBT? There is no fixed number. One full mock per week, plus topic tests and a careful review, works better than many mocks without review.