Deduction guide
Deduction questions ask what must be true given the premises — syllogisms, conditional chains and quantifier logic. The answer is what the premises force, never merely what they make possible.
The method
- Ask what must follow, not what could be true.
- Use the contrapositive: 'if P then Q' also gives you 'if not Q then not P'.
- Draw quantifier statements ('all', 'some', 'none') as overlapping sets.
- For ordering relations, lay the items on a line and read off what is fixed.
Deduction questions: prove necessity, not likelihood
Deduction asks what must follow if every premise is accepted. Translate conditional statements into arrows and category statements into sets. Then test whether the proposed conclusion holds in every arrangement permitted by the premises.
To attack an answer, try to construct one valid counterexample. If the premises can all be true while the conclusion is false, the conclusion does not follow. Never reverse an implication: all A are B does not mean all B are A.
Final check before you answer
- Ignore whether the premises sound realistic; treat them as rules.
- Chain only implications that point in the correct direction.
- Look for a counterexample before accepting a conclusion as necessary.
Syllogism questions in aptitude tests
A reasoning syllogism gives you two or more premises and asks what must follow. Treat category words as sets and ignore whether the statements sound realistic. For example:
All analysts are graduates. No graduates are interns.
Conclusion: No analysts are interns.
The conclusion follows because the entire analyst set sits inside graduates, while the graduate and intern sets cannot overlap. Syllogism aptitude questions reward this strict set logic—not assumptions about the people described.
Common mistakes
- The converse error — reading 'if P then Q' as 'if Q then P'.
- Reading 'some' as 'most' or 'all'.
- Existential-import slips — assuming a category has members when it needn't.
- Flipping the direction of an ordering relation.
Worked examples
Three real questions from this category, moving from easy to hard, with the verified answer and the exact evidence that decides each one.
Example 1: easy
Every chilled churn is moved to the loading bay. Every churn in the loading bay is weighed by the dairyman.
Conclusion: Every chilled churn is weighed by the dairyman. Does the conclusion follow from the premises?
Verified answer: A — Follows
The evidence
Every chilled churn is moved to the loading bay. Every churn in the loading bay is weighed by the dairyman.
The highlighted text decides the answer.
What to notice
Two one-way rules chain through 'the loading bay'.
The rule
Hypothetical syllogism: (chilled -> bay), (bay -> weighed) gives (chilled -> weighed).
Why the others are wrong
- B — Does Not Follow: The two rules chain cleanly; rejecting the link drops premise 2.
Example 2: medium
If a tram enters the depot, its brakes are tested. If a tram's brakes are tested, the logbook is signed.
Which conclusion follows from the premises?
Verified answer: D — If a tram enters the depot, the logbook is signed.
The evidence
If a tram enters the depot, its brakes are tested. If a tram's brakes are tested, the logbook is signed.
The highlighted text decides the answer.
What to notice
Two one-way rules chain forward through 'brakes are tested'; the foils run the rules backwards.
The rule
Hypothetical syllogism: (enter -> brakes tested), (brakes tested -> logbook signed) gives (enter -> logbook signed).
Why the others are wrong
- A — If a tram's brakes are tested, the tram entered the depot.: Reverses premise 1; a brake test need not mean the tram entered the depot.
- B — If the logbook is signed, the tram entered the depot.: Affirms the consequent; a signed logbook has other causes.
- C — Every tram enters the depot.: Asserts every tram enters the depot, which no premise states.
- E — If a tram does not enter the depot, the logbook is not signed.: Denies the antecedent; not entering does not block a signature.
Example 3: hard
If a parcel misses the depot cut-off, it is routed to the late bay. If a parcel is routed to the late bay, it travels on the relief van, which reaches the hub after midnight. If a parcel reaches the hub after midnight, its delivery is rescheduled to the next working day.
Which conclusion follows from the premises?
Verified answer: A — If a parcel misses the depot cut-off, its delivery is rescheduled to the next working day.
The evidence
If a parcel misses the depot cut-off, it is routed to the late bay. If a parcel is routed to the late bay, it travels on the relief van, which reaches the hub after midnight. If a parcel reaches the hub after midnight, its delivery is rescheduled to the next working day.
The highlighted text decides the answer.
What to notice
A three-link chain ringed by an affirm-consequent and a deny-antecedent trap.
The rule
Iterated hypothetical syllogism from cut-off through late bay and relief van to rescheduling.
Why the others are wrong
- B — If a parcel makes the depot cut-off, its delivery is not rescheduled.: Denies the antecedent; making the cut-off does not guarantee no reschedule.
- C — If a parcel travels on the relief van, it missed the depot cut-off.: Reverses premise 2; the relief van does not prove a missed cut-off.
- D — Every parcel reaches the hub after midnight.: Asserts every parcel arrives after midnight, which no premise states.
- E — If a parcel's delivery is rescheduled, it missed the depot cut-off.: Affirms the consequent; a reschedule can have other causes.
Frequently asked questions
What's the single most useful move?
The contrapositive. It is always valid and it rescues most conditional-chain questions where the converse would trap you.
Practise this now
See more worked examplesStart practising — free
Use deduction in Watson-Glaser-style practice
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