Yes. Sudoku is completely free in your browser on ArcadeWhale. No download, no sign-up and no payment — pick a difficulty and start.

No. Sudoku involves no arithmetic at all. The digits 1 to 9 are simply nine distinct symbols, and the puzzle is pure logic about where each symbol can and cannot go. The game would work identically with nine colours or nine shapes.

Fill the nine-by-nine grid so that every row, every column and every three-by-three box contains each digit from 1 to 9 exactly once. Some digits are given at the start, and the rest are deduced from them.

Notes mode lets you write small pencil marks into a cell listing the digits that could still go there. It is how most people solve harder puzzles, because it takes the candidate list out of your head and puts it on the grid where patterns become visible.

A properly constructed Sudoku has exactly one solution, and every puzzle here is built that way. That is what makes guessing unnecessary — any cell you cannot yet fill can be worked out by logic from the cells you can.