Select a cell, then tap a digit on the on-screen number pad. Use Erase to clear a cell you placed. Cells given at the start are fixed.

The Highlight conflicts option marks digits that repeat in a row, column or box in red as you play. You can turn it off for a stricter challenge.

There are four difficulty levels. Beginner starts with 48 filled cells and 12 minutes, Easy has 40 and 9 minutes, Medium has 32 and 6 minutes, and Hard has 24 and 4 minutes. If the countdown reaches zero, the attempt ends.

There are no pencil marks or hints in this version, so work out candidates in your head or on paper.

Each level has a base score: 400 for Beginner, 700 for Easy, 1,100 for Medium and 1,800 for Hard. You lose 40 points for every mistake, meaning a digit that does not match the solution, and you gain one point for every second left on the clock.

Your best score is saved in your browser under the username you pick. If you sign in, personal bests can also sync to your account.

Start with the easiest wins. A naked single is a cell where only one digit is still possible. A hidden single is a digit that has only one place left to go in its row, column or box. Scan for both repeatedly, because each new digit unlocks more.

When singles run out, look for naked pairs: two cells in a unit that share the same two candidates, which means those two digits can be removed from every other cell in that unit. Then try pointing pairs, where a digit in a box is confined to one row or column and so is removed from the rest of that line.

The game first builds a complete valid grid with a backtracking solver. It then removes cells in random order until the target number of clues is reached, and checks that the puzzle still has exactly one solution. If it does not, it tries again. Because every puzzle is verified, you never need to guess; there is always a logical path.

The mathematical minimum for a Sudoku with a single solution is 17 clues, a result proved in 2012. Most published puzzles have more, because they are easier and more pleasant to solve.

The modern puzzle was designed by an American, Howard Garns, and published in 1979 in a Dell puzzle magazine under the name Number Place. It became popular in Japan, where the publisher Nikoli gave it the name Sudoku in the 1980s, and it became a worldwide craze in the mid-2000s.

• On Hard you only have four minutes, so do not hunt for advanced patterns until you have exhausted every single.
• Fill in digits you are certain of first, because every placed digit narrows the rest.
• Mistakes cost 40 points each, which is more than half a minute of time bonus, so accuracy matters more than raw speed.
• If you are stuck, move to a different box. A fresh area often has a single you missed.