Well surprising because 21 is quite "logical" in my opinion (and easier than 20...) - this is the beauty of this game : each player has its own way of playing it.
Hint : gur serr fdhner yrsg gb gur "9" vf * irel * hfrshy
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Thanks A12r13 - easier than 20?! Gah.. your comment helped, though indirectly - somehow it triggered a breakthrough on how to get a 6 and myself into the right position to open the first lock. I'm now able to get to almost thru but with a 6 where I need a 9... EDIT: Ah, got it! Yup, so obvious and simple when you get there..