Solving Sudoku puzzles made easy (part 0)

Here's the 'challenging' Sudoku from this morning's Metro. I normally do these on the Tube in the morning, a time of day when I am not particularly awake, so I have to use some sort of system, which I will explain in today's series of posts. I've time-stamped them in reverse order so that you can read top-to-bottom.
Full series here.

Solving Sudoku puzzles made easy (part 1)

First do the easy ones, i.e. the 1 in the bottom-right box and then the 3s, which seem fairly straightforward.
That leaves you with only place where 5 can go in the bottom-left box.
The 7s are a bit trickier, but if you mentally shade the cells in the top-right and middle-right boxes where a 7 can't go, you see that they must be on the top row in each box, so in the top-left and centre boxes, they must be on the bottom row.
Full series here.

Solving Sudoku puzzles made easy (part 2)

Next you write down all the numbers missing from the first two columns (2,4,6,8,9 and 2,4,8,9). I chose those two because those columns are already reasonably well 'populated', you can start with whichever rows or columns already have the most numbers in them.

The only number that can go in column two, row six is 8. There's then only one place the 8 can go in the top-right box
We know that the 6 in the first column can't be in the bottom-left box so there's only one place that can go as well.

For future reference, make a note where the 2,4 and 9 could go in the bottom-left box.
Full series here.

Solving Sudoku puzzles made easy (part 3)

There's only one place that the 4 can go in the top-left box.
Make a note in column two where the 2 and 9 could go.
Make a note in middle-left box where the 2 and 9 could go.
Let's attack row three next, so write down the missing numbers 1,2 and 9 to the right of the puzzle.
Full series here.

Solving Sudoku puzzles made easy (part 4)

There's only one place that the 4 can go in the top row, so do that.

We're now doing row three, missing numbers 1,2 and 9.
There's only one place for the 2 (it can't go in the middle-top box).
That leaves you with only one place in row three where the 9 can go (because there's already a 9 in column six in the middle-bottom box), so you then do the 1 as well.

Having noted (in part 3) where various 2s and 9s could go, and having just entered a 2 in the top-left box, we know that the cell marked "2,9" must be a 9, so do that, then we also know where the 2 goes in the middle-left box, so do that, and we also know where the 9 goes in that box.

That leaves us with 5 and 8 to insert in the top-middle box. There's already an 8 in column six (middle-bottom box) so do the 8 and then the 5.

Next, we'll attack row 2, so write down the missing numbers 1,2 and 6 in the margin at the left.

Full series here.

Solving Sudoku puzzles made easy (part 5)

We're doing row two, missing numbers 1,2 and 6.
There's only one place the 2 can go, so do that, that leaves you only one place where the 1 can go, once you've done that, plop in the 6 in the top-left box.
So there's then only one place in the top-left box where the 5 can go.
Next, attack column four. There's only one place the 9 can go, and only one place the 7 can go (actually we could have done the 7 earlier, ah well).
Then look at column six. The only missing numbers are 2 and 6, so going back to column four, the 2 has to go in where it does.
We can have a stab at row one as well, so write down 6,7 and 9 in the margin.

Full series here.

Solving Sudoku puzzles made easy (part 6)

We're doing column six, which has 2 and 6 missing. The 6 can't be in the middle-bottom box, so put it in the centre box and do the 2 in the middle-bottom box. (I could have done this before the previous step, same difference).
Then do row five, we have 5 and 8 missing, so write them down in the margin.
There's already a 5 in column seven, so put the 8 in the middle-right box and then do the 5.
That leaves you with only one place in the centre box where the 8 can go.
Then note where the 1 and 4 could go in the centre box.
Next, we'll attack row seven, so write down 1,2,4,5 in the margin.
To tidy up row one, write in where the 6,7 and 9 could go in the top-right box.

Full series here.

Solving Sudoku puzzles made easy (part 7)

In the previous part, we noted where 1 and 4 could go in the centre box, i.e. in row six, so we know without even looking that the 5 must go in the only empty space left in row six.
That then leaves you with only one place where the 5 can go in the bottom-right box.
In row six, we now only have 1,2 and 4 missing. There's only one place that the 2 can go, so do that, and note where the 1 and 4 could go for future reference.
While we're at it, we can also write down where the 1,6 and 7 could go in the middle-right box for future reference.
In the next part, we'll attack row nine (the bottom row), so write down the missing numbers 2,4,6,9 in the margin.
Full series here.

Solving Sudoku puzzles made easy (part 8)

We're doing row nine (the bottom row), missing numbers 2,4,6,9.
Handily, there's only one place the 4 can go so enter that in the middle-bottom box.
We noted earlier on that the very bottom-left cell was either 2,4 or 9. It can't be a 2 because by now there's already a 2 in that bottom-left box. Neither can it be a 4 (there's already a 4 in row nine), so it must be a 9.
So the cell above that, which we had previously marked "2,4,9" (way back in part 1)' must be a 4.

To finish off row nine, we need to put the 2 in the only place it can go and then put the 6 in the bottom-right cell.
Full series here.

Solving Sudoku puzzles made easy (part 9)

By now, you should have four cells in rows six and seven which are marked "1,4" (i.e. we had previously established that these cells could be either 1 or 4).
Having entered a four in the bottom-middle box, it's now party time.
The cell marked "1,4" in the bottom-middle box must be a 1.
So the one above that must be a 4.
So the one to the left if that must be a 1.
So the top-middle cell in the bottom-right box must be a 4.
Finish off the bottom-middle box with a 5.
This leaves us 1 and 9 missing from row eight. Do the 9 first and then plug in 1.
Full series here.

Solving Sudoku puzzles made easy (part 10)

All we have left to complete are the top rows in the top-right box and the middle-right box. As we have pre-entered possibles in these cells, the rest is on automatic.

Let's start with the 9 we already have in the bottom-right box.
The cell in row one, column eight is marked "6,9", it can no longer be 9 so make it 6.
The cell to the left of that 6 was marked "6,7" so make it 7 and finish off the top-right box with a 9.

The middle cell in the top row of the middle-right box is marked "1,6", it can no longer be a 6 so mark it 1.
The cell to the left of it is marked "1,6,7". It can no longer be a 1 (look at the 1 in row eight) or a 7 (look at the 7 in row one) so make it a 6.
Finish off with a 7 in the top-right cell of the middle-right box.
Job done.
Full series here.