This is a math problem by the way. ...4 -> 16 -> 37 -> 58 -> ? -> 145 -> 42 -> 20 -> ... The process loops around and repeats on 4. There is a similar relationship between all the numbers. The ? is the 1 number that will make this entire thing true and continue on. Two hints for you: 1) Each one is linear, so 4 becomes 16, 16 becomes 37, etc 2) Break out each number as digits. So 145 becomes 1, 4, 5