Logical Reasoning: Constraint Puzzles and Valid Deduction
What the section measures
SPJAT logical reasoning mixes analytical puzzles with deductive items: linear and circular arrangements, blood relations, direction sense, syllogisms, number and letter series, coding–decoding, and short assumption–conclusion–inference passages. No outside knowledge is needed. Marks go to candidates who fix constraints in a stable order instead of redrawing the grid repeatedly.
Core rules
- Arrangements: draw fixed slots first, place the most restrictive clue, then apply "immediately left/right" and "not adjacent" conditions.
- Circular seating: n people give n! linear orders but (n−1)! circular orders; clockwise and anticlockwise mirror images count as the same unless stated otherwise.
- Blood relations: build one chain from a single reference person before drawing anything; convert "maternal uncle" and "sister-in-law" into explicit links.
- Direction sense: track net displacement on the x and y axes separately, then use Pythagoras for the shortest path.
- Syllogisms: only three relations matter - all, none, some. "Some A are B" is reversible; "all A are B" is not.
- Series: test differences, then ratios, then alternating, prime or square patterns before committing to one rule.
Worked example
All engineers on the team are analysts. Some analysts are managers. Does it follow that some engineers are managers?
No. "Some analysts are managers" does not tie those managers to the engineer subset. The engineers may sit entirely inside the analyst group while the overlapping managers fall outside it. The conclusion is not forced. The only safe deduction is the conversion of the first premise: some analysts are engineers.
Traps that cost marks
- Reading "some A are B" as also implying "some A are not B".
- Treating "no A is B" as compatible with "some A is B".
- Importing outside facts into an assumption question; the right assumption must be necessary, not merely plausible.
- Committing a layout to paper before every clue has been scanned.
- Missing that a conclusion can be true but still not follow.