Mensa IQ Logical Deduction Problems Flashcards

All roses are flowers. Some flowers fade quickly. What can be deduced?

Answer: Some roses may fade quickly

Since all roses are flowers, and some flowers fade quickly, it's possible that some of those flowers that fade quickly are roses. However, we cannot be certain — we can only say some roses MAY fade quickly.

If it rains, the ground gets wet. The ground is not wet. What can you conclude?

Answer: It is not raining

This is modus tollens: If P then Q. Not Q. Therefore not P. Since rain always causes wet ground, and the ground isn't wet, it cannot be raining. This is one of the fundamental rules of logical deduction.

Alice is taller than Bob. Bob is taller than Charlie. What is true?

Answer: Alice is taller than Charlie

Height comparisons are transitive: if A > B and B > C, then A > C. Since Alice is taller than Bob, and Bob is taller than Charlie, Alice must be taller than Charlie.

No fish can fly. All salmon are fish. What can be deduced about salmon?

Answer: No salmon can fly

This is a classic syllogism. Since all salmon are fish, and no fish can fly, it follows that no salmon can fly. The property of 'cannot fly' transfers from the larger category (fish) to the subset (salmon).

If A implies B, and B implies C, what can we conclude?

Answer: A implies C

This is the transitivity of implication (hypothetical syllogism). If A→B and B→C, then A→C. If A is true, B must be true, and if B is true, C must be true. Therefore if A is true, C must be true.

In a room of 5 people, everyone shakes hands with everyone else exactly once. How many handshakes occur?

Answer: 10

Each of the 5 people shakes hands with 4 others, giving 5×4=20 hand-involvements, but each handshake involves 2 people, so divide by 2: 20/2 = 10. The formula is C(5,2) = 5!/(2!×3!) = 10.