OK, so nearly all of them seem impossible at first glance but 21 is killing me, particularly after I found 20 to be relatively easy.. and particularly as it doesn't seem to given anyone else any trouble!.. Hint please?..
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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..