The gimmick behind Everest is that it operates in base 11 modular arithmetic. Like a clock face, except with 11 places instead of 12, and each number has only one digit; 10 is represented as X, and 11 is the same as 0. 0 minus 1 is X, so X *is* negative one. Adding 9 is the same as subtracting 2.
Anyway, I reached level 10 or 11 doing just mental math, but it was slow and frustrating. Then I realized I could use pencil and paper to help me so I jotted some things down and eventually came up with a couple tools:
I can’t figure out a way to geometrically represent the rule added in level 7, but there are a few easy things to keep in mind:
A|B + C|D = A+C|B+D A|B * C|0 = AC|BC A|B * 0|C = (-B)C|AC (in particular, 0|A * 0|A = -A) A|B * C|D = (A|B * C|0) + (A|B * 0|D)
I mean, it’s pretty obvious what’s going on algebraically. I just wish there was some kind of chart I could draw to make it easier at a glance.
