Don’t waste your time looking for unique candidates such as numbers that can only fit in one particular cell of a row, box or column.

Start off immediately by filling in all possible candidates in every cell. If you’re working with hard copy, use a pencil (or buy a smart phone and download an app), if you’re doing the puzzle on your smartphone or computer, the app (I prefer Sudoku – Free Classic Sudoku Puzzles by dailyinnovation.biz) will provide you with the means of filling in these candidates.

I used to recheck my work before I moved on to the next step as there’s nothing more frustrating than being 80% through a puzzle and finding that you made a mistake right at the beginning which prevents the solution of the puzzle. However, I’ve found that the above app does not allow me to enter an invalid candidate and it also shades all areas relevant to a particular cell (i.e. the column, row and box that it is in) – switch on “Highlight areas” via the setup gearwheel. This makes it easy to see which numbers are not candidates.

Once you have filled in all candidates (it takes me about 9 minutes), switch off “Highlight areas” via the setup gearwheel, make sure that the pencil is switched on and always work with the pencil on unless you want to enter a solution in a particular cell.

If you have the right app, you can now touch any of the numbers “1” on the puzzle and all of the 1’s will show up in a colour, then do 2, then 3 etc all the way to 9. At each number look for the patterns below

Check each row at a time. If there’s a number (such as the green 6’s) that can only be in a particular box of that row, then eliminate that number (the orange 6’s) from the other two rows in the box. In doing this, you will spot the unique candidates that you could have waste time looking for earlier.

At the same time you will recognise pairs of pairs (the green 3 4 pairs) and can eliminate those numbers (the orange 3s and 4s) from the rest of the row, since if B1 is a 3, then H1 is a 4 and if B1 is a 4 then H1 is a 3, so neither of those numbers can be in any other cells in Row 1.

Compete all nine rows, then do the same for columns, again recognising pairs of pairs.

Then go on to boxes. If any number can be in only one particular row or column within a box, then eliminate that number from the rest of the row or column respectively. You’ll get to the point where you can check rows, boxes and columns simultaneously.

Again, this is an opportunity to recognise pairs of pairs within a box (see the green 2s and 4s above). If you find a pair of pairs, you can eliminate those numbers from all other cells within the box (the orange cells above).

Now, the chances are very good that after all of this, the puzzle has changed sufficiently that it’s worth doing the whole sequence again. But this time, whizz through, looking only for sole candidates in any row, box, or column.

If that doesn’t finish the puzzle, start with the number 1 again, looking for all of the above patterns and also looking for triples. They’re fairly easy to spot, but probably best kept for now rather than trying to check simultaneously with the above, especially because they’ve most likely only occurred during the above processes. The green triples below enable you to eliminate the triple numbers from all other cells in that row, column or box.

You can also look for X-wings (Google these if you don’t know how to spot them), which aren’t difficult to see and usually occur at least once at expert level.

Repeat 1 through 9 until you can find no more adjustments.

If that doesn’t solve the problem, now is the time to get into the advanced techniques of Swordfish, XY-Wing, and Unique rectangle. You can find these via Google, but will probably have gone over your target time of about 30 minutes. 

 May speed be with you! Enjoy!