i tried it a couple of times.. and think i have it. you always seem to be able to reduce the numbers to x amount of 9's at the end. so, for example, if the number you have in the end is 4950 and you chooce 5.. that leaves you to input 490 to it. thus, it seeks what number it has to add to make it divisable by 9, which in this case would be 5.. which was the number you had in paper

another case: original number = 2645, mixed up number = 6425, after reduction... 3780 the value of the numbers by themselves, is, 2 +7 and 8+1.. and again we can count the missing number

that is also the reason why you were not allowed to chooce 0