Brain Teasers · Twelve Balls and a Scale
12 Balls 3 Weighings Puzzle - Find the Odd Ball
Twelve balls, one heavier or lighter, and three weighings on a balance to find it and say which.
What is the twelve balls puzzle?
A balance has three outcomes - left heavy, right heavy or level - so each weighing is a three-way question. Three weighings can tell apart 27 cases.
Twelve balls that could each be heavy or light give 24 cases, which fits, but only if every weighing splits the remaining cases as evenly as possible.
How it works
Twelve identical balls, one fake that is heavier or lighter, and a balance you can use three times. The story shows the scale tipping left, right and staying level, then asks how to find the fake and its direction. You pick one of four answers. Then you step through the answer and the breakdown.
How scoring works
This teaser checks your answer. A wrong pick is crossed out. You get two tries before the answer opens. A correct first try earns a ticket; a second try or opening the answer early is recorded but earns nothing. The story teasers share one lifetime unlock for free users.
The key insight
Weigh 4 against 4. Whatever happens, eight cases are left. Then mix suspects with balls you know are genuine, and move some suspects to the other pan, so the second weighing leaves at most three cases and the third settles it.
The full step-by-step solution lives at /brain-teasers/twelve-balls/solution.
A worked example
If 1-4 is heavier than 5-8, weigh 1, 2, 5 against 3, 6, 9. Left heavy means 1 heavy, 2 heavy or 6 light. Right heavy means 3 heavy or 5 light. Level means 4 heavy, 7 light or 8 light. One more weighing picks out the ball in each case.
Common mistakes
• Starting with 6 against 6: it can never balance, so it only splits the cases two ways.
• Treating each weighing as a yes-or-no question and concluding it can't be done.
• Naming the fake ball but not whether it is heavy or light.
Why interviews test this
Saying the 24-versus-27 count out loud before any weighing is what interviewers listen for. The written solution is at /brain-teasers/twelve-balls/solution.
Play Twelve Balls and a Scale on Outcry
Twelve Balls and a Scale is free to play on Outcry, in the Brain Teasers section, and every run is timed and scored. Read the guide once, then play Twelve Balls and a Scale and check your score against the strategy above. The guide book has a written guide for every other game, and the practice hub and mock assessments put the same skills under a real screen's clock.