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.
<u>Example:</u>
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) <em><u>IF</u></em> 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.
I hope this answered your question. If you have any more questions feel free to ask away at Brainly.