The 100 Coins | Folj.com

5. The 100 Coins

There are 10 sets of 10 coins. You know how much the coins should weigh. You know all the coins in one set of ten are exactly a hundredth of an ounce off, making the entire set of ten coins a tenth of an ounce off. You also know that all the other coins weight the correct amount. You are allowed to use an extremely accurate digital weighing machine only once.

How do you determine which set of 10 coins is faulty?

Added 1 January 2007

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Hint:

You can weigh as few or as many of the ten coins from each set as you choose.

Solution:

One coin from the first set is placed on the scale along with two from the second set etc... If the weight is off by one hundredth of an ounce then it is the first set that is faulty, if the weight is off by two hundred of an ounce then it is the second set which is faulty, etc...

Comments (1)

Anonymous 25 May 2013

Another way to solve this would be to place all the bags on the scale at the same time. Pull each bag off one by one watching the weight go down. Say bag 7 is the bag that weighs a tenth less. If each bag weighs 1 lb minus bag 7, the total weight will be 9.9 lbs.

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