This is the famous 3n+1 equation. In this equation, which is as-of-yet unsolved, any number can be put into this equation and will always reach 1 as the final answer. Maybe YOU could put a random number into this equation and find out which number could be the number that doesn't reach 1 as a final answer!
If the number is even, then it will be divided by 2. If the number is odd, then it will multiply by three and be added by 1.
