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Given the following position, what colour are the pieces and what was the last move played?

[FEN "B1KRN3/PP6/2QK1R2/8/8/8/8/8 w - - 0 1"]
2
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The difficult part of this puzzle is the realization that the pawn on b7 can actually be black. When this is established, the rest of the puzzle can be figured out without too much difficulty. Thus, if we assume that the pawn on b7 is black, what does this entail for the other pieces? Well:

  • The bishop on a8 must be white, since it must've gotten there by means of pawn promotion. This further implies that the path to a8 must've been cleared for a white pawn at some point. Hence, the pawn on a7 must be white as well.
  • Next, we realize that the rook on d8 and the queen on c6 must be of the same color (otherwise both kings would be in check). Moreover, these must be of the color opposing that of the king on c8; otherwise the king on d6 would be in check by the queen and the rook, which cannot be the result of a legal move on the previous turn. This, in turn, proves that the king on d6 is of the same color as all of the heavy pieces and the knight, since it couldn't possibly be in check.
  • Finally, we determine the color of the king on c8. If it were white, then the rooks, and queen must be black by the previous point. But there is no legal move on the previous turn that black could've made to achieve this. So the king has to be black, which leads us to the realization that the last move played must've been cxd8=R, since the move must've been a discovered check (Note that this implies that the pawn on b7 must indeed be black, since the arguments concerning the heavy pieces are not dependent on the color of this pawn).

So, this must be the solution (the knight is indeed the only possible piece that could be on d8 in the initial position, as pointed out by James Martin in the comments):

[FEN "B1knN3/PpP5/2QK1R2/8/8/8/8/8 w - - 0 1"]

1.cxd8=R
5
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    you didn't seem to actually say it, but it seems to me that b7 not only can be black, must be black, since otherwise it would also check the black king and all three checks can't come from one move. – ilkkachu Dec 17 '20 at 15:36
  • @ilkkachu indeed, you're right. If the pawn on b7 is white, then it means that the king on c8 is white as well. Then, by the same line of reasoning I used in my post, all the heavy pieces must be black and we run into the problem of the position being illegal. This is what I got stuck on before I realized that the pawn on b7 could be black. I guess the first insight to this puzzle should really be that the king on c8 is the one who is in check. – Scounged Dec 17 '20 at 16:33
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    Can the thing on d8 actually be a bishop, as you mention? Black doesn't seem to have a possible previous move in that case. I think it must be a knight, and furthermore Black has just played Ne6-d8 (or Ne6xd8). – James Martin Dec 17 '20 at 19:33
  • @JamesMartin Good point, I didn't consider that at all. I'll update the answer accordingly. – Scounged Dec 17 '20 at 21:53
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    The "normal" way to deduce it without assuming the colour of an arbitrary piece is to notice that the two kings are "checked" by both the c6 queen and d8 rook. The only way this makes sense is if both queen and rook are the same colour giving a double check, and this is only possible with a discovered promotion to a rook. All the other pieces' colours follow from needing to avoid checking the kings and to allow the bishop on a8 to be promoted. – Remellion Dec 18 '20 at 0:52

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