TL;DR
The 12 Balls Weighing Puzzle: A canonical quantitative trading interview question at olympiad difficulty. Commonly asked at Jane Street, SIG, Optiver, HRT.
By Valenke Exam Prep Team·Last updated 2026-06-01
olympiadLogic & Constraints
The 12 Balls Weighing Puzzle
Asked at: Jane Street, SIG, Optiver, HRT
Problem
You have 12 identical-looking balls. Exactly one is defective (either heavier or lighter than the rest — you don't know which). Using a balance scale and at most 3 weighings, identify the defective ball AND determine whether it is heavier or lighter.
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