In front of you, there are 9 coins. They all look absolutely identical, but one of the coins is fake. However, you know that the fake coin is lighter than the rest, and in front of you is a balance scale. What is the least number of weightings you can use to find the counterfeit coin?
* When we multiply three numbers, we will get the prime numbers.
* The difference between the second and the first number is equal to the third and second.