Place the numbers 1-9 in the boxes so that the difference of each pair of numbers joined by a line is at least three. The number 8 has already been placed.
(In reply to solution by reasoning
by Ady TZIDON)
Ady, you have reached the same solution which Charlie and I put forward, but we both used exhaustive search programs. It seems that there might be an approach by reasoning (which some call "analytic"). Of course, trial-and-error probably eventually would find the solutions, but "analytic" usually portends some systematic shortcuts. I'd be interested to see how you came to the solution.