Series & Coding-Decoding: Spot the Pattern, Score Fast

Learn to hear the hidden "rhythm" behind number and letter series, convert letters into positions using the EJOTY trick, and crack coding-decoding shifts — with step-by-step worked examples built for RRB NTPC speed and accuracy.

By the PadhoDost Team · 📖 7 min read · Updated 4 August 2026

Part of Railway Exams (RRB) prep

🧠 Every pattern has a rhythm

Picture a railway platform board flashing train numbers: 12002, 12004, 12006... Even before the next one appears, your ear already expects 12008. Your brain quietly caught the rhythm (+2). Series and coding questions work exactly like this — once you spot the hidden rhythm behind the numbers or letters, the answer almost announces itself. Padho, Dost — let's train your eyes to hear that rhythm every single time.

Number Series: Crack the Hidden Rule

A number series is a line of numbers arranged by a secret rule. Your only job is to find that rule and use it to fill the blank. The good news: the rule is almost always one of just a few common families. Once you know which families to look for, your eyes start catching them in seconds.

Pattern typeExampleThe rule
Add / subtract a fixed number4, 9, 14, 19, ...+5 each time
Multiply / divide3, 6, 12, 24, ...×2 each time
Squares or cubes1, 4, 9, 16, ...n² for n = 1, 2, 3, 4...
Growing gap2, 5, 10, 17, ...gaps go +3, +5, +7
Mixed (multiply then add)5, 11, 23, 47, ...×2 then +1
Alternate / woven series1, 2, 4, 3, 9, 4, ...two series mixed together

How to solve any number series

  1. 1Read the whole series once — note if the numbers rise or fall, and whether slowly or fast.
  2. 2Find the differences between consecutive terms (2nd − 1st, 3rd − 2nd, and so on).
  3. 3If the differences are all equal, the rule is 'add or subtract a fixed number' — done.
  4. 4If the differences grow, study the pattern of those differences (do they go +2, +4, +6? or do they double?).
  5. 5If neither fits, try ratios (divide each term by the one before it) for a ×2 or ×3 rule, or check for squares and cubes.
  6. 6Confirm your rule works across at least 3 gaps, then apply it to find the missing term.

📝 Worked example: 5, 11, 23, 47, ?

Question: Find the next number → 5, 11, 23, 47, ?

Step 1 — Check the gaps: 11 − 5 = 6, 23 − 11 = 12, 47 − 23 = 24. The gaps 6, 12, 24 are doubling, not fixed.

Step 2 — A doubling gap hints at a ×2 rule. Test plain ×2: 5 × 2 = 10, but we need 11. So try ×2 then +1.

Step 3 — Verify: 5×2+1 = 11 ✓, 11×2+1 = 23 ✓, 23×2+1 = 47 ✓. The rule holds every time.

Step 4 — Apply to the last term: 47 × 2 + 1 = 95.

Answer: 95.

💡 Two-check shortcut: first look at the differences (Are they fixed? Growing?). If the differences don't settle into a clean pattern, check the ratios (divide each term by the previous one) for a ×2 or ×3 rule. Most NTPC series crack open with just these two checks in under 20 seconds.

Letter Series & Alphabet Positions

Letter series look scary, but they are just number series wearing a disguise. Give every letter its position number — A = 1, B = 2, C = 3, all the way to Z = 26 — and suddenly C, F, I, L... becomes 3, 6, 9, 12. Now it is a plain number series you already know how to solve. Convert the answer back into a letter at the end, and you are done.

ℹ️ EJOTY memory hook: E = 5, J = 10, O = 15, T = 20, Y = 25. Memorise just these 5 anchor points. To find any letter's number, jump from the nearest anchor instead of counting all the way from A. Need R? T = 20, count back 2 → R = 18. This tiny trick saves you precious seconds on every letter question.

📝 Worked example: C, F, I, L, O, ?

Question: C, F, I, L, O, ? — what comes next?

Step 1 — Write the position numbers: C = 3, F = 6, I = 9, L = 12, O = 15.

Step 2 — Find the gap: each term jumps +3 (3 → 6 → 9 → 12 → 15).

Step 3 — Next position = 15 + 3 = 18.

Step 4 — Letter at position 18 = R (using EJOTY: T = 20, so 18 is two letters before T → R).

Answer: R.

Coding-Decoding

In coding-decoding, a word is written in a secret 'code' by shifting or rearranging its letters. The common types are: (1) letter-shift — each letter moves forward or backward by a fixed step, like +1 or +2; (2) position-number coding — letters are replaced by their alphabet numbers; and (3) reversing the word. Your task is to spot exactly how the given word turned into its code, then apply that same trick to the new word.

📝 Worked example: MANGO → NBOHP

Question: If MANGO is coded as NBOHP, how is APPLE coded in the same code?

Step 1 — Line up the letters: M→N, A→B, N→O, G→H, O→P.

Step 2 — Every code letter is +1 position (simply the next letter of the alphabet). That is the rule.

Step 3 — Apply +1 to APPLE: A→B, P→Q, P→Q, L→M, E→F.

Answer: BQQMF.

⚠️ Common traps to avoid: (1) Miscounting the alphabet under pressure — always jump from EJOTY anchors instead of counting from A. (2) Mixing up forward and backward shifts — decide the direction first, then apply it to every letter. (3) In 'growing gap' series, many students see the first gap and wrongly assume it stays fixed — always confirm at least 3 gaps before locking in a rule.

Exam-day game plan

  • ✓CBT-1 gives you 30 reasoning questions in roughly 27 minutes — series and coding are fast, high-scoring picks, so grab them early.
  • ✓For number series: check differences first, then ratios, then squares and cubes.
  • ✓For letter series: convert letters to position numbers, solve as a number series, then convert back.
  • ✓For coding: line up the words letter-by-letter and identify the shift (+1, +2, −1, reverse, etc.).
  • ✓Negative marking is −1/3 per wrong answer — if two different rules seem to fit and you cannot confirm one, skipping is often safer than a blind guess.
  • ✓Practise 10–15 of these daily; with repetition, pattern-spotting becomes almost instant.

⚡ Quick check

If FACE is coded as GBDF, then how is HAND coded in the same code?

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