If you add all the substrings of 891, the sum is 198, which is the reverse of 891:
891 → 8 + 9 + 1 + 89 + 91 = 198
This is the only 3-digit number that has this property.
There is also one 4-digit number with the same property. Can you find this number?
As I said above, there is only a single solution. The answer is 2931:
2931 → 2 + 9 + 3 + 1 + 29 + 93 + 31 + 293 + 931 = 1392
To solve this kind of problem, you may resort to trial and error but it could get cumbersome. A simple Python or C++ code can do the trick for this.
Now, you may try to figure out if there is a 5-digit number solution.