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InterviewSkill

Puzzles Interview Questions

Logic puzzles and constraint-solving patterns commonly used in analytical interview rounds.

5 questions
Puzzles

You have 8 balls and one is heavier. How can you find it in two weighings?medium

Type
problem-solving
Topic
have-8-balls-one-heavier-find-it-two-weighings
Frequency
common
Tags
have, 8, balls, one, heavier, find, it, two, weighings
Answer

Split into 3, 3, and 2. Weigh 3 against 3, then use the result to narrow to one ball.

Explanation

If the first two groups balance, the heavy ball is among the remaining two and one weighing finds it. If one group is heavier, weigh one ball from that group against another; if balanced, the third is heavier.

Hint: reason from constraints before calculating.
Follow-upHow would the strategy change with 9 balls?

Three switches control three bulbs in another room. How do you match them with one visit?easy

Type
problem-solving
Topic
three-switches-control-three-bulbs-another-room-match-them-with-one-visi
Frequency
common
Tags
three, switches, control, three, bulbs, another, room, match, them, with, one, visi
Answer

Use heat as state: turn one on, one on then off, and leave one off.

Explanation

Turn switch A on for a few minutes, turn it off, turn switch B on, and leave C off. In the bulb room, the lit bulb is B, the warm unlit bulb is A, and the cold unlit bulb is C.

Hint: reason from constraints before calculating.
Follow-upWhat assumption does this puzzle rely on?

How do you measure exactly 4 liters with 3-liter and 5-liter jugs?medium

Type
problem-solving
Topic
measure-exactly-4-liters-with-3-liter-5-liter-jugs
Frequency
common
Tags
measure, exactly, 4, liters, with, 3, liter, 5, liter, jugs
Answer

Fill the 5-liter jug, pour into the 3-liter jug, empty the 3-liter jug, then transfer the remaining 2 liters and refill to reach 4.

Explanation

After filling 5 and pouring 3, 2 liters remain. Empty the 3-liter jug, pour the 2 liters into it, fill the 5-liter jug again, then pour 1 liter into the 3-liter jug to fill it. The 5-liter jug now has 4 liters.

Hint: reason from constraints before calculating.
Follow-upHow do you generalize jug puzzles using gcd?

A clock shows 3:15. What is the angle between the hands?easy

Type
problem-solving
Topic
clock-shows-3-15-angle-hands
Frequency
common
Tags
clock, shows, 3, 15, angle, hands
Answer

The angle is 7.5 degrees.

Explanation

At 3:15, the minute hand is at 90 degrees. The hour hand is not exactly at 3; it has moved a quarter of the way toward 4, so it is at 97.5 degrees. Difference is 7.5 degrees.

Hint: reason from constraints before calculating.
Follow-upWhy is 0 degrees a common wrong answer?

How do you solve seating arrangement puzzles systematically?hard

Type
problem-solving
Topic
solve-seating-arrangement-puzzles-systematically
Frequency
common
Tags
solve, seating, arrangement, puzzles, systematically
Answer

Draw fixed positions, place the strongest constraints first, and keep branches separate.

Explanation

Anchor one person or position to remove rotational ambiguity. Then apply direct constraints, mark impossible slots, and branch only when necessary. Good notation matters more than speed.

Hint: reason from constraints before calculating.
Follow-upHow do you avoid rechecking the same invalid branch?