Aptitude · Reasoning

Number series, letter series & coding-decoding

Pattern questions look endless, but nearly all of them come from about six patterns. Learn to check them in a fixed order and you'll stop guessing.

Your win todayCrack a series or a code in under 45 seconds by checking patterns in a fixed order.

Number series: the 4-check ladder

Work left to right and stop at the first check that fits every term. Differences solve the largest share, so always start there.

Letters: think in numbers

Convert letters to positions, solve it as a number series, then convert back. Remember EJOTY and you can find any position in two seconds.

Example: A, C, F, J, O, ? → 1, 3, 6, 10, 15 → gaps +2, +3, +4, +5 → next +6 → 21 → U.

Coding-decoding: the five codes

Type	Example rule	Then
Shift	MANGO → OCPIQ (each letter +2)	APPLE → CRRNG
Opposite letter	TIGER → GRTVI (27 − position)	LION → ORLM
Reverse + shift	CAT → reverse TAC → +1 → UBD	DOG → HPE
Letters → numbers	CAT = 3+1+20 = 24	DOG = 4+15+7 = 26
Sentence code	Compare sentences: the shared word ↔ the shared code	see below

How to work: write the positions under both the word and its code, subtract letter by letter, and look for one constant or a mirror (sum = 27). Practice bank: IndiaBIX letter series, number series.

Sentence code: which code means "sweet"?

tee see pee = drink fruit juice
see kee lee = juice is sweet
lee ree mee = he is intelligent

Sentences 1 & 2 share "juice" and "see" → see = juice. Sentences 2 & 3 share "is" and "lee" → lee = is. What's left in sentence 2: kee = sweet.

Retrieval check

2, 6, 12, 20, 30, ?

5, 10, 8, 16, 14, ?

A, C, F, J, O, ?

MANGO is coded OCPIQ. What is APPLE?

What is the opposite letter of H?

Type it

1, 2, 6, 24, 120, ?

TIGER is coded GRTVI. What is LION?

2, 5, 10, 17, 26, ?

Your turn

Write A–Z with positions from memory using EJOTY. Time yourself (target: under 60 s).
Do 10 questions from IndiaBIX Number Series, naming which ladder step solved each one.
Do 10 questions from IndiaBIX Letter Series.
Invent one code of each of the five types and ask a friend (or your teacher) to crack it.
Memorise squares to 20² and cubes to 10³. These make step ③ instant.

Go deeper

Primary source: IndiaBIX: Number Series. It's the most widely used practice bank for Indian placement reasoning tests, with explanations for each answer.

Related: Lesson 10: AP (an AP is just a series with constant differences) · Aptitude cheat sheet · Aptitude question bank

💬 Ask your teacher: "give me 10 mixed series and codes, timed" or "blood relations / direction sense?"