Consider this structure being imposed upon the "Olympic Rings" to create 15 regions.
Place one number from 1 to 15 in each region so that the middle top ring (Black) has a total of Z + 2 while the other 4 total Z each.
Ring Values:
1. A B F G [Z] 2. B C D G H I J K [Z+2] (Black) 3. D E K L [Z] 4. F G H I M N [Z] 5. I J K L N O [Z]Note: Olympic Rings has fewer overlaps.