First published by Money Metals here: https://www.moneymetals.com/news/2017/10/03/gold-coin-dilemma-001170

There are five identical bags of gold, and each contains ten gold coins. However, one of the five bags contains fake gold. The real gold, fake gold, and five bags appear identical, except the coins of fake gold each weigh 1.1 ounces, and the real gold coins each weigh 1 ounce. You have an accurate digital scale and CAN USE IT ONLY ONCE.