Addition Sudoku (PuzzleMadness)

Every row, column and 3x3 box holds 1 to 9 once each. The grey groups are sums set out as you would write them on paper: the top rows hold the numbers being added and the bottom row holds the answer. A single column of three cells reads as a one-digit sum, such as 3 + 1 = 4, while a block two cells wide reads its rows as two-digit numbers, so 47 above 32 above 79 means 47 + 32 = 79. In the site's first example the sums are 7 + 1 = 8, 6 + 5 + 2 = 13 and 3 + 5 = 8.

A two-digit total starts with 1 whenever the numbers added are single digits, and wide groups bring in carrying, which links the columns of the sum. That gives a different flavour from killer sudoku, where you only know a cage's total, not how its digits are arranged.

The grid works like every sudoku on PuzzleMadness: number buttons above and below it, an eraser, and a pencil mode for candidate digits. On a keyboard, the number keys enter digits, the arrow keys move between cells, Ctrl, Shift or Alt with a number adds a pencil mark, and Delete or Backspace clears a cell.

PuzzleMadness is a free puzzle website run by Dewi, a software developer who started out making games for the PlayStation 2, Nintendo Wii and DS and now builds the site as a hobby. It carries dozens of puzzle types, from classic sudoku to newer logic grids, and each one gets its own dated puzzle every day at each difficulty. Addition Sudoku comes in Easy, Medium, Hard and Tough versions plus a Mixed page, and the same day's Addition Sudoku puzzle is shared by everyone who opens it.

The site checks your grid as you go and tells you when it is finished; it does not publish solutions. Each completed puzzle earns a score, and scores feed a monthly and an all-time Addition Sudoku leaderboard, where only your best attempt at a puzzle counts. Older days stay in the archive, so a missed puzzle can still be played later and still counts toward the current month. Playing needs no account; a free registration gives you a username on the leaderboards and a cloud save. The pages carry adverts.