There are five identical looking bags of gold coins. Four of the bags contain coins made of real gold and one of the bags contains coins made of fool’s gold.

All of the coins in all of the bags look identical. They are the same size, color and shape. The only difference is their weight. Real gold coins weigh 10 grams and coins made of fool’s gold weigh 11 grams.

You have an accurate scale that you can use only one time to determine which bag contains the fool’s gold. How do you do it?

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Take one coin from the first bag, two from the second bag, three from the third bag, four from the fourth bag, and five from the fifth bag. If the weight on the scale ends in 1 you know its the first bag, 2 the second, 3 the third, 4 fourth, and 5 the fifth bag.