[math-fun] 31 lines of zero (mod 15) puzzle.

Here's an orchard of 31 lines of three on 15 points. That might be maximal according to A003035. https://i.imgur.com/w18qs7R.jpg Arrange the fifteen numbers -7 to 7 in the disks so that the (mod 15) sum of all lines is zero. --Ed Pegg Jr