CDS (Combined Defence Services) Exam Reasoning and Mental Ability Syllabus

List the four cardinal and four ordinal compass directions.

Cardinal: North, East, South, West. Ordinal (intermediate): North-East, South-East, South-West, North-West.

In direction sense, if you face North and turn 90 degrees clockwise, which way do you face? And anticlockwise?

Clockwise 90 degrees -> East; anticlockwise 90 degrees -> West.

What is the shortest-distance formula between start and end points in a direction problem?

Use the Pythagorean theorem on the net horizontal and net vertical displacements: distance = sqrt(horizontal^2 + vertical^2).

At sunrise your shadow falls in which direction, and at sunset?

At sunrise (sun in the East) the shadow falls toward the West; at sunset (sun in the West) the shadow falls toward the East. At noon the shadow points North (in the Northern Hemisphere).

In linear ranking, if a person is 7th from the left and 9th from the right, how many people are in the row?

Total = (left position) + (right position) - 1 = 7 + 9 - 1 = 15.

In ranking, what is the formula to find a person's position from the other end?

Position from other end = (Total number) - (position from given end) + 1.

In a class rank problem, if A is 15th from the top and there are 40 students, what is A's rank from the bottom?

40 - 15 + 1 = 26th from the bottom.

What are the two main seating arrangement types tested in reasoning?

Linear arrangement (people in a row facing one direction) and circular arrangement (people around a table facing the centre or outward).

In a circular arrangement facing the centre, how do left and right appear compared to a linear row?

They are reversed: a person's left and right are opposite to the observer's, because they face inward toward the centre.

What is a syllogism?

A logical argument with two premises and a conclusion, where the conclusion must necessarily follow from the premises based on the relationship between categories.

Name the four standard categorical proposition types used in syllogisms (A, E, I, O).

A = Universal Affirmative (All S are P); E = Universal Negative (No S is P); I = Particular Affirmative (Some S are P); O = Particular Negative (Some S are not P).

What is the most reliable method to solve syllogism questions?

Draw Venn diagrams for the premises; a conclusion is valid only if it holds true in EVERY possible diagram arrangement of the premises.

In syllogisms, can a conclusion be valid from two particular (Some) premises?

No. Two particular premises (I/O) cannot yield a definite conclusion; at least one universal premise is required.

In syllogisms, what is a 'complementary pair' (either-or conclusion)?

When two conclusions are contradictory (e.g., 'Some A are B' and 'No A is B') and individually neither is definite but together they exhaust all cases, the answer is 'Either I or II follows.'

From 'All cats are dogs' and 'All dogs are animals', what valid conclusion follows?

All cats are animals (and 'Some animals are cats').

In Statement-and-Assumptions questions, what is an 'assumption'?

Something taken for granted or supposed - an unstated idea that must be true for the statement/action to make sense. A valid assumption is implicit, not directly stated.

What is the key test for whether an assumption is valid in Statement-Assumption questions?

Negate the assumption: if negating it makes the statement collapse or become illogical, the assumption is implicit (valid). Avoid choosing obvious, over-strong, or already-stated ideas.

In Statement-and-Arguments questions, what makes an argument 'strong' versus 'weak'?

A strong argument is directly related to the statement, addresses it logically, and is significant/practical. A weak argument is irrelevant, trivial, ambiguous, or based on personal opinion/assumption.

In Statement-and-Arguments, should an argument's strength depend on whether you personally agree with it?

No. Strength is judged by relevance and logical connection to the statement, not by personal opinion or factual desirability.

What is the Cause-and-Effect relationship in reasoning questions?

It tests whether one event (the cause) directly produces another event (the effect); you must decide if one statement is the cause, the other the effect, both are effects of a common cause, or they are independent.

List the typical answer options in a two-statement Cause-and-Effect question.

(1) Statement I is cause, II is effect; (2) II is cause, I is effect; (3) both are independent causes; (4) both are effects of independent causes; (5) both are effects of a common cause.

In Cause-and-Effect, how can you usually tell the cause from the effect by timing?

The cause occurs before the effect in time; the earlier event is generally the cause and the later/resulting event is the effect.

What is a 'figure series' in non-verbal reasoning?

A sequence of figures changing by a consistent rule (rotation, addition/removal of elements, shading, movement); the task is to choose the figure that continues the pattern.

In a figure analogy (non-verbal), what must the answer figure satisfy?

The change from figure C to the answer must be exactly the same transformation that turns figure A into figure B (same rotation, reflection, or element change).

What are the common transformations to look for in figure series/analogy problems?

Rotation (clockwise/anticlockwise by a fixed angle), reflection, increase/decrease in number of elements, change in shading, and movement of elements within the figure.

What is a mirror image in reasoning?

The lateral inversion of a figure as seen in a vertical mirror placed beside it: left and right are interchanged while top and bottom stay the same.

How does a mirror image affect letters like b, p, and the word 'BOX'?

Mirror reversal flips left-right: 'b' appears as 'd', 'p' as 'q', and 'BOX' reads as the laterally inverted form 'XOB' with each letter itself reversed.

What is a water image in reasoning?

The vertical (upside-down) reflection of a figure as if mirrored in water below it: top and bottom are interchanged while left and right stay the same.

In paper folding and cutting problems, how do you determine the result when the paper is unfolded?

Each cut/hole is reproduced symmetrically across every fold line; unfold step by step, mirroring the punched holes about each crease to get the final pattern.

In a standard dice/cube, what is true about the sum of opposite faces, and how many faces meet at a corner?

On a standard die, opposite faces sum to 7 (1-6, 2-5, 3-4). Three faces meet at each corner of a cube, and a cube has 6 faces, 8 corners, and 12 edges.