Answer:
Only 1 Time
Step-by-step explanation:
Since you know that one coin is fake and that the coin is lighter than the rest , then you would only need to weigh all the coins together 1 time to ensure that you will find the fake coin. We can determine this by doing the following.
Example:
Lets say we have 100 coins that weigh 1 gr each. Meaning that the total weight of the 100 coins would be (100coins*1gr = 100gr) IF there are no fake coins in the batch.
Knowing this we can weigh the entire batch 1 time and if the total weight is less than 100gr we know that the fake coin is in that batch.
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