The 100 Coins - Can you solve it?

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 (4)

Anonymous 30 May 2010

The method for finding the faulty set of coins is not fully explained. You can weigh one coin from each set to identify the faulty set based on the weight discrepancy.

Anonymous 3 June 2010

The explanation for identifying the faulty set of coins is incomplete. You should take one coin from each set and weigh them to determine which set is faulty based on the weight difference.

Anonymous 12 June 2010

The method described for determining the faulty set of coins is not the most efficient. You can take one coin from each of the ten sets and weigh them all together to find the faulty set.

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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