Rules are from Takeya Saikachi’s blog. In Puzzle ?77, digits are from 1 to 7.
Background: Takeya is the owner of the puzzle idea. If you click the link, you can find two of Takeya’s puzzles. Puzzle ?77 is my fifth puzzle in this type. I prepared 2 of them for UKPA Puzzle Blogs part, and the other 2 is for Zagreb Open 2013 set.