Surface Sum
Two overlapping groups of runs differ by exactly one cell — subtracting their clue totals determines that cell's digit outright.
How it works
- 1Add the two down clues: 4 + 7 = 11. That is the exact total of the four tinted cells.
- 2Now add the two across clues: 4 + 10 = 14. Those two runs cover the same four cells — and one cell more, the one hanging off the end of the lower row.
- 3Everything in the tinted block cancels between the two sums, so the leftover cell is 14 − 11 = 3.
- 4Not one tinted cell is pinned down and it does not matter: this is clue arithmetic over a region whose two directions don't quite line up. Hunt for it wherever a row runs one cell past the columns beside it.
- Cells that establish the pattern
- Where the conclusion lands