Mental Ability: Number Series, Coding, Analogy, and Odd One Out

Mental Ability (Reasoning) questions appear in every Kerala PSC exam — typically 5-10 questions. These are scoring questions because they don’t require memorisation, just pattern recognition. This guide covers the most common question types with worked examples.

1. Number Series

Task: Find the missing number or the next number in a sequence.

Common patterns

Addition series (constant difference):

• 3, 7, 11, 15, ? → Difference = +4 → Answer: 19

Multiplication series:

• 2, 6, 18, 54, ? → Each × 3 → Answer: 162

Square series:

• 1, 4, 9, 16, 25, ? → 1², 2², 3², 4², 5² → Answer: 36 (6²)

Cube series:

• 1, 8, 27, 64, ? → 1³, 2³, 3³, 4³ → Answer: 125 (5³)

Alternating operations:

• 2, 5, 11, 23, ? → ×2+1, ×2+1 → (2×2+1=5), (5×2+1=11), (11×2+1=23), (23×2+1=47)

Increasing difference:

• 1, 2, 4, 7, 11, 16, ? → Differences: +1, +2, +3, +4, +5 → Answer: 22 (+6)

Fibonacci pattern:

• 1, 1, 2, 3, 5, 8, ? → Each number = sum of previous two → Answer: 13

Practice problems

1. 5, 10, 20, 40, ? → ×2 each time → 80
2. 3, 6, 11, 18, 27, ? → Differences: +3, +5, +7, +9 → +11 → 38
3. 1, 4, 9, 16, 25, 36, ? → Perfect squares → 49
4. 2, 3, 5, 7, 11, 13, ? → Prime numbers → 17
5. 100, 95, 85, 70, ? → Differences: −5, −10, −15 → −20 → 50

2. Letter Series and Coding

Alphabet position values (memorise this)

A=1	B=2	C=3	D=4	E=5	F=6	G=7	H=8	I=9
J=10	K=11	L=12	M=13	N=14	O=15	P=16	Q=17	R=18
S=19	T=20	U=21	V=22	W=23	X=24	Y=25	Z=26

Letter series patterns

Forward skip (+2):

• A, C, E, G, ? → Skip one letter each → I

Reverse series:

• Z, X, V, T, ? → Skip one backward → R

Mixed pattern:

• AB, DE, GH, ? → Each pair starts 3 letters after previous → JK

Coding-Decoding

Type 1: Letter shift If COME = DPNF (each letter +1), then GONE = HPOF

Type 2: Reverse coding If HELP = PLEH (reversed), then COME = EMOC

Type 3: Number coding If A=1, B=2… then FACE = 6+1+3+5 = 15

Practice problems

1. If MANGO is coded as OCPIQ (+2 each), how is APPLE coded? → CRRNG
2. B, D, H, N, ? → Differences: +2, +4, +6 → +8 → V
3. AZ, BY, CX, ? → First goes forward, second goes backward → DW

3. Analogy

Task: A : B :: C : ? (A is related to B as C is related to what?)

Common relationship types

Relationship	Example
Opposite	Hot : Cold :: Big : Small
Part-whole	Page : Book :: Room : House
Worker-tool	Carpenter : Saw :: Painter : Brush
Male-female	King : Queen :: Bull : Cow
Product-source	Butter : Milk :: Oil : Seed
Country-capital	India : Delhi :: Japan : Tokyo
Country-currency	India : Rupee :: USA : Dollar
Young one	Dog : Puppy :: Cat : Kitten
Measurement	Thermometer : Temperature :: Barometer : Pressure

Practice problems

1. Doctor : Patient :: Teacher : ? → Student
2. Pen : Writer :: Needle : ? → Tailor
3. 4 : 16 :: 5 : ? → 4²=16, 5²= 25
4. Mango : Fruit :: Rose : ? → Flower
5. Fish : Gill :: Human : ? → Lung

4. Odd One Out

Task: Find the item that doesn’t belong to the group.

Strategy

1. Identify the common property — all might be fruits, even numbers, or planets
2. The odd one lacks this property

Examples

1. Apple, Mango, Potato, Orange → Potato (vegetable, rest are fruits)
2. 2, 5, 7, 11, 9 → 9 (not prime; 9=3×3)
3. Venus, Mars, Moon, Jupiter → Moon (satellite, rest are planets)
4. January, March, June, July → June (30 days, rest have 31)
5. Square, Rectangle, Triangle, Cube → Cube (3D, rest are 2D)

5. Direction Sense

The compass:

         N
         |
    W ---+--- E
         |
         S

Rules:

• Turn right from North → East
• Turn left from North → West
• Opposite of North → South
• Turn right from East → South

Example

“A man walks 5 km North, then turns right and walks 3 km, then turns right again and walks 5 km. How far is he from the starting point?”

Solution: North 5 km → Right (East) 3 km → Right (South) 5 km. He’s back at the same latitude as start, 3 km East. Answer: 3 km (East of starting point).

6. Blood Relations

Key relationships

Relationship	Meaning
Father’s/Mother’s father	Grandfather
Father’s/Mother’s mother	Grandmother
Father’s brother	Uncle (Paternal)
Mother’s brother	Uncle (Maternal)
Father’s/Mother’s sister	Aunt
Brother’s/Sister’s son	Nephew
Brother’s/Sister’s daughter	Niece
Son’s wife	Daughter-in-law
Daughter’s husband	Son-in-law

Example

“Pointing to a man, a woman says ‘His mother is the only daughter of my mother.’ How is the woman related to the man?”

Solution: “Only daughter of my mother” = the woman herself. So the man’s mother is the woman. Answer: Mother.

7. Calendar and Clock Problems

Days in months (memory aid)

30 days: April, June, September, November (“30 days hath September…”) 31 days: January, March, May, July, August, October, December 28/29 days: February

Leap year rules

1. Divisible by 4 → leap year
2. BUT divisible by 100 → NOT leap year
3. BUT divisible by 400 → IS leap year

Examples: 2024 → leap (÷4). 1900 → NOT leap (÷100 but not ÷400). 2000 → leap (÷400).

Odd days concept

Period	Odd days
1 ordinary year	1 odd day (365 = 52 weeks + 1)
1 leap year	2 odd days (366 = 52 weeks + 2)
100 years	5 odd days
200 years	3 odd days
300 years	1 odd day
400 years	0 odd days

Clock angle formula

Angle between hour and minute hand = |30H − 5.5M|

Where H = hour, M = minutes

Example: At 3:20, angle = |30(3) − 5.5(20)| = |90 − 110| = 20°

Exam Strategy for Mental Ability

1. Do these questions LAST in the exam — after completing GK questions which are pure recall
2. Draw diagrams for direction and seating arrangement questions
3. Write out the alphabet with position numbers at the top of your rough sheet before starting
4. Skip and return — if a question takes more than 60 seconds, skip it and come back
5. Check for negative marking — if yes, don’t guess; if no, attempt everything

Patterns based on Kerala PSC previous year question analysis (2018-2024). All question types appear regularly in LDC, Last Grade, and Graduate Level papers.