Calcudoku

How to play

Calcudoku is played on a four-by-four grid. Like Sudoku, every row and every column must contain each digit 1 through 4 exactly once. On top of that, the grid is divided into "cages" — groups of bordered cells, each carrying a small clue in one corner: a target number and an operator. The digits inside a cage must combine under that operator to reach the target — add up to it, multiply to it, or for a two-cell cage, subtract or divide to it in whichever order gives a positive result. Tap a cell, then tap a digit below the board to fill it in. A cage with no operator shown is a single cell whose digit is simply given outright.

Working it out

Single-cell cages are free digits — fill them in first, since a whole row or column is instantly one digit lighter. From there, a cage with a very restrictive target is the next best one to look at: on a board where every digit is 1-4, a two-cell cage reading "12×" can only be 3 and 4 — no other pair multiplies that high — while a much smaller target like "3+" leaves several pairs open and tells you far less on its own.

Just like Sudoku, once you know a digit can only go in one cell in a given row or column, place it — even if you haven't fully worked out the cage it belongs to yet. The row/column rule and the cage rule are two separate sources of information about the same grid, and combining them is most of what solving a Calcudoku actually is.

Turn on "Highlight mistakes" below the board if you'd like a filled-in digit marked in red the moment it doesn't belong in the finished solution.