Making an assumption:-
That if a substitute digit is itself in the lookup table, it will be replaced again.
Digit 2 3 4 5 6 7 8 Substitute digit 3 4 9 1 7 8 0
The Summation becomes:
999 + 1 = 1000 because:
2->3->4->9,
3->4->9,
4->9,
5->1,
5->1,
6->7->8->0,
7->8->0,
8->0
Process:
As the question states, if you follow the guidelines, it should lead you towards the answer
First, as mentioned in the question, there is one possible summation. It must be 999 + 1 = 1000 as a 3 digit number plus a 1 digit number must equal a 4 digit number, and the first digit of the 4 digit number has to be the same as the 1 digit number.
Then, knowing that 6,7,8 must equal 0 we can first assign any one of those digits the substitute digit of zero, lets choose 8.
Since 0 is now used (and the question states the substitute digits must be unique) in order for 6 or 7 to equal 0, the only substitute digit we can assign is 8 (since 8 = 0).
This same logic is then applied to 2,3,4 since they all need to equal 9
