Fake coins  logic puzzle
Suppose you have five bags filled with the same type of coins and
a weigthing machine. One of this bags is completely filled with fake coins, the other bags contain only real ones. A real coin weights
10 gram and a fake one 11 gram. What is the minimal number of times you need to use the weighting machine in order to find the bag with the fake coins? 
Answer
With using the weighting machine only once it is possible to find the bag with the fake coins.
