"If X were 204 and the sequence were 42162 then what would be the logical approach?"
Similar to Sudoku, SUMurai is about enumerating possibilities and applying constraints.
Unlike Sudoku, it requires some basic addition skills. In many ways, it is easier than Sudoku
but can quickly confuse people: "Where do I start?"
Given your example, one can start by ruling out 421 as a possible split (bigger than X).
Therefore valid splits including 4 can only be 4 or 42. Here the example is not tricky enough
to justify going further than that. It would be overkill.
At this point, most humans can just see that the solution is 42 + 162.
In more complex examples, it is really a matter of being conscious about the constraints
that this or that split impose on the remaining sequence.
Let me highlight that, except for a very special case in the game, 3-digit splits are, for now, forbidden.
2-digit splits are more than enough to give interesting challenges.
See the examples below to be convinced of that.
They are random samples taken from the training page of the app (I was able to
solve them within a minute. They are not too hard but more than enough for a beginner.)
If you manage to solve them (which would be a feat, given that I just introduced the game to you)
you will get out of it with a more clear answer to your question
than all the babbling I did above
X = 77 for 15683
X = 79 for 532168
X = 88 for 191414941
Bonus: The MEAN version
Here, X is not the sum, but the mean of the splits. For example, with X=27 and 15723, the solution is 3 splits 1 57 23 which add up to 81; 81 / 3 = 27. So, you have to take into account the number of your splits. In fact, compared to the SUM version, this is where a big deal of logic come into play. In particular, you have to analyze the sequence to make an informed guess about which multiple of X you should go after.
X = 31 for 9579
X = 14 for 884653
X = 42 for 11398917
There are many more interesting variants in the game.
If you have an Android phone, just go and get the game. If not, install an emulator such as BlueStacks and play on your computer.