@turboman: However, consider that in this puzzle they already know the last three numbers, and the nature of the puzzle is such that no number can be used twice.
So there are 7 possible numbers, twice, so 7^2 = 49. However, within that set no number can be used twice, so remove the 7 "doubles" from within that, and our final answer is 42 - unless I'm dumb and have missed something out.
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