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Thank you for the hints. I got ityoda wrote:Spoiler!
Feedback for the improvement of the book:
I (finally!) got the answer, but the weird, back-and-front 5th character is very misleading and confusing.
This one is the first one that was actually really kind of bad. Not so much a puzzle to "logic" out, but just a stupid guessing exercise. Disappointing one.
Actually don’t like this one at all, makes no sense comparing to all others because the principle changes as you go down so how you suppose to follow it.. and yes-3d letter in not recognizable