The answer is not the point.
Nobody is scoring whether you happen to already know the twelve-coins answer. They are watching how you decompose a problem you've never seen, what you do when your first idea doesn't work, and whether you can explain your reasoning clearly enough for someone else to follow it.
Before you start
If you got stuck halfway through, would you say so out loud — or go quiet and hope the interviewer waits?
When your first approach to a puzzle turns out wrong, do you narrate the correction or start over silently?
Could you explain your solution to someone who hasn't seen the puzzle before, in a way they'd actually follow?
01 / Ten puzzles
Work each one before reading what it's scoring.
- 1
You have eight identical-looking balls; one is heavier. Find it in two balance-scale weighings.
What it's scoring
Splitting into groups of three rather than halves, and using the imbalance (or balance) of the first weighing to eliminate two-thirds of the possibilities before the second.
- 2
You have two ropes, each taking exactly 60 minutes to burn but at non-uniform rates. Measure 45 minutes.
What it's scoring
Realising that lighting a rope from both ends halves its remaining burn time — the insight the whole puzzle hinges on.
- 3
Three switches are outside a room and one bulb is inside. You may enter the room only once. Identify which switch controls the bulb.
What it's scoring
Using a third variable — heat — once you realise light alone can only distinguish two states, not three.
- 4
You have 12 coins and one counterfeit that may be heavier or lighter. What information must a weighing strategy preserve after each step?
What it's scoring
Explaining information preservation explicitly, not just producing a weighing sequence from memory.
- 5
Why are manhole covers usually circular? Give more than one operational or engineering reason.
What it's scoring
Generating multiple independent reasons (can't fall through the hole, easiest shape to roll, no orientation to align) rather than stopping at the first one.
- 6
A lily patch doubles in size every day and covers a pond in 30 days. On which day is it half-covered? Explain why intuition gets this wrong.
What it's scoring
Naming exponential growth explicitly and explaining why the intuitive halfway guess (day 15) is wrong — this is really a test of exponential reasoning, not arithmetic.
- 7
You have 100 lockers, and on the nth pass you toggle every nth locker. Which lockers remain open, and why?
What it's scoring
Connecting the answer to perfect squares — a locker is toggled once per divisor it has, and only perfect squares have an odd number of divisors.
- 8
A clock loses five minutes every hour. How would you calculate when it next shows the correct time on a 12-hour dial?
What it's scoring
Setting up the problem as accumulated drift versus the full 12-hour cycle, rather than guessing or trial-and-error.
- 9
Construct a decision tree for whether to launch a product when three uncertain events must happen in sequence, each with a given probability.
What it's scoring
Correctly multiplying sequential probabilities rather than adding them, and reasoning about expected value, not just the most-likely single path.
- 10
The answer is 42. Generate three different business questions for which 42 could be a reasonable answer, stating the units each time.
What it's scoring
Producing genuinely different question types (a count, a percentage, a rate) rather than three trivial variations of the same question.
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