There's a type of puzzle where you're in front of a grid (two colors, 6x6) that is a cellular automaton. There's a grid you can interact with, and one that shows what you need to obtain. So you have to:
Figure out the rules (basically variants of the Game of Life)
Find a state that leads to the given grid
Finding the rules isn't that complicated, but I have troubles finding something for stage 2.
I could give you the one I'm stuck on, but I'd be stuck on the next one, what I need is a method or some kind of lead to try and crack it.
I'm thinking of filling a grid, counting the number of neighbors, and basically tick white or black when I'm sure, and then cover more and more of the grid like in Minesweeper, but it doesn't seem to work, I'm stuck at step one with no clue.
TL;DR: Given a Game of Life grid at state n, how can I find a possible n-1 grid?
EDIT: Here's a screenshot of the puzzle, maybe it'll help. The rules are: "A cell will be red at the next iteration if it touches at least 5 red cells (including itself)". The grid is toroidal.
The input is the same as the goal because I found it to be good starting point.
Here's the source. (in French, but you don't really need to read anything. The cellular automaton puzzles are on col 0, row 1-2)