Calcudoku

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Calcudoku is a math sudoku with cages. A tray holds a square grid of sunk squares. Every row and every column has to hold each number once, from 1 up to the width of the tray. Raised walls divide the tray into cages. The number in a cage corner is the target, and the sign beside it says how the numbers in that cage reach it.

Take away and divide only ever appear on a cage of two squares, and either order counts: a cage marked 2 divide holds 4 and 2, or 2 and 4. A cage of one square has no sign, so it simply is its number. A cage of three or four squares adds or multiplies. Two squares in one cage may hold the same number when they are in different rows and different columns.

Three sizes. A 4 by 4 tray uses 1 to 4, a 5 by 5 tray uses 1 to 5, and a 6 by 6 tray uses 1 to 6. A wider tray has more cages and more squares to fill, so it takes longer.

Choose a square, then press a number or tap one under the tray. A number a row or column already holds is refused, and the page says which one is in the way. That refusal is counted. Nothing else is refused. A number that repeats nothing in its row or column is taken, even when it is not the answer, and most of those turn nothing red. Press Check to mark every square that does not match the answer. Check is not counted.

Notes hold the numbers you are still choosing between. Turn notes on, then a number key writes a small mark in the square instead of filling it. Filling the square clears its notes.

A cage turns green when its numbers make its target. It turns red when they cannot. That happens as soon as a cage that adds has passed its target, or a cage that multiplies can no longer divide into it, or a full cage is wrong. Undo takes back one move at a time. Clear the tray empties it without changing the puzzle, and Undo brings it back.

Hint first points to useful cells or clues. Press Explain and apply to see the reason and apply that one step. An applied hint counts once.

The clock starts on your first move and stops when the last cage turns green. It pauses while the tab is in the background. A tray you leave part way through is kept on this browser and comes back when you press Start again.

The statistics page compares trays of the same size. Every saved tray is a solved one, so what changes there is the time you took.

The result says how long this tray took you. It is not a test, and it says nothing about any other puzzle.

The lesson starts with the 3 by 3 magic square, then shows which cages to fill first. Learn to start with the small cages