Counterfeit Coin Sack Puzzle - One Weighing, Played

How it works

Five sacks of 100 coins. Real coins weigh 10 g, and every coin in the one fake sack weighs 9 g. A digital scale gives one reading. The story asks how to find the fake sack. 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

Take k coins from sack k: 15 coins, 150 g if all real. If sack k is fake, k coins are each 1 g light, so the reading is 150 - k.

The full step-by-step solution lives at /brain-teasers/counterfeit-coins/solution.

A worked example

A reading of 147 g means sack 3. If several sacks could be fake, take 1, 2, 4, 8 and 16 coins and read the shortfall in binary.

Common mistakes

• One coin from each sack: the pile is 1 g light whatever sack is fake.

• Weighing whole sacks one at a time - that needs more than one reading.

• Taking the same number from two sacks, so two answers give the same reading.

Why interviews test this

The idea is to make each possible answer look different. Expect the binary follow-up. The written solution is at /brain-teasers/counterfeit-coins/solution.

Play The Counterfeit Coins on Outcry

The Counterfeit Coins is free to play on Outcry, in the Brain Teasers section, and every run is timed and scored. Read the guide once, then play The Counterfeit Coins 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.