Thank you for creating them!
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fwiw I’m 16k (on OGS, and don’t have a rank anywhere else) and I went through the whole collection the other day, up to 16k1, because I noticed the new collection by chance. I got all of them right on the first try with some careful thinking, except for this one https://online-go.com/puzzle/21271 which I still want to look at carefully later
Today I saw this thread and went back, and I got 16k2 wrong after missing the bulky 5 shape. I got 16k3 and 15k1 correct on the first try. So, they seem like they are labeled well.
Always enjoy your puzzles Mark, thanks for making them 
even the ones technically rated below my rank were fun to solve.
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Puzzle 36 has a slightly different board position in the solution (the problem version has an outside stone in atari which is not there in the solution)
At an SDK level, Play Go at online-go.com! | OGS is a bit unsatisfying because there are multiple solutions, but there are still higher-level things which can be considered.
Solutions
I first checked that they all worked, and then that they all left the same amount of ko threats.
Since I think they do, I conclude that C is the best move as if Black ends up living because of a ko fight elsewhere on the board, C reduces the group’s territory whereas A and B don’t.
Ironically, C is the only solution that rejects the theme of the tsumego.
Another thing is that in this later position, the throw-in at X is unnecessary. Similar to the earlier position, F1 is better for Black since it reduces the group if it ends up living, whereas the throw-in doesn’t.
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Thanks for reminding me… I had forgotten the problem but something stuck in my head since today I found one solution on the first try without too much difficult reading. I should revisit problems more, it’s a nice feeling 
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