Since there is no nice neat formula for this sort of exercise, a "big long solution" is pretty much all there is, being the explanation of the "intelligent guess-n-check" required for solving this.
For instance, you see that you are adding three four-digit numbers and getting a four-digit number. This tells you that M + S + N (plus whatever "carry" there might be from the tens' to the hundreds' place) must be 9 or less. So this puts a cap on that sum.
You know that, whatever E is, it is the ones' digit of 3N. So look at the options for 3N:
. . . . .3, 6, 9, 12, 15, 18, 21, 24, 27
Then work through the options, working with 3N, E, and 3O. And so forth.
Yes, it's long and involved. The point (I think) is that you learn to work logically, so that you can find a solution with a minimal amount of effort.
Eliz.