I guess, this may become quite natural with practice: eliminating contradictory variants. So it may be well a matter of first glance… (since it's really a matte

I guess, this may become quite natural with practice: eliminating contradictory variants. So it may be well a matter of first glance… (since it's really a matter of tautology and observations like "M = 1" aren't really a matter of thought, as soon as you've encountered this once.) Then you're left with just a few variants to actual think about and maybe try them to see, if they work. Which should become also more effortless with practice. Some of this "second layer" may reveal itself as soon as we've "cracked" the outer shell. So, I guess, you may be able to look at this and say, "well, E must be either 5 or 6 … and, well, it's actually 5", eventually, especially, as well known subproblems become some kind of idiom and we've learned to look for such possible vectors of attack.