Can you explain how you arrived at 42 Ohms? I understand the formula for a single delta config Rxy = (RA+RB)RC/(RA+RB+RC) so do I start on the left and "cascade" the calculated value over to the delta config to the right as I solve for it?
Once I saw a similar puzzle, but it had an infinite number of triangles to the left side (or was it made with squares? I don´t remember, it was many years ago). This type of topologies ussualy make very good puzzles.
Perhaps this is the case, which of course has a diferent answer than the finite one we were solving.