The last digit (not "number") could be absolutely anything, in the absence of further information. It might, for example just be the last digit of a random measurement; or it could be a check digit following one of many possible schemes; or this could represent one of many possible patterns.
I have no idea what "can be from two, three, four positions" means, or in what sense you could "try" anything, or why you wouldn't try "0"; but presumably you know more than you have told us about the number.
I also suspect you have no idea what a differential equation is. This is not one.