Faulty Coin Puzzle

Faulty coins:

This type of faulty coins/cigarette questions are very often asked in interviews. This was the question asked in my Interview too. Therefore, everyone learn to answer it.

Question:

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?

Answer:

Take 1 coin from first, 2 coins from second, 3 coins from third and so on till 10 coins from tenth set.

Weigh the coins, which you have taken from 10 sets in a manner described above.

Check the weight-

If you find that weight is off by one hundredth of an ounce, the first set is faulty.

If you find that weight is off by two hundredth of an ounce then the second set is faulty.

So on…