zedoku

last possible number

level 1 - super easy

look at what's already in the row, column, and box — the missing number is the answer. this is one of the simplest sudoku techniques, perfect for beginners.

the candy bag analogy

imagine you have a bag with 9 candies, numbered 1 through 9.

your friends come over and each takes some candies:

  • friend from your row takes candies: 1, 2, 3, 4, 5, 7, 8, 9
  • friend from your column takes candies: 2, 3, 5, 6, 7, 8, 9, 1
  • friend from your box takes candies: 1, 2, 3, 4, 5, 6, 8, 9

which candy is left? it must be the one that none of your friends took!

the answer is 6 — the only number not in any of the three groups.

6
only one left!

example 1: one empty cell

look at this puzzle. there's only one empty cell remaining. can you figure out what goes there?

5
3
4
6
7
8
9
1
2
6
7
2
1
9
5
3
4
8
1
9
8
3
4
2
5
6
7
8
5
9
7
6
1
4
2
3
4
2
6
8
5
3
7
9
1
7
1
3
9
2
4
8
5
9
6
1
5
3
7
2
8
4
2
8
7
4
1
9
6
3
5
3
4
5
2
8
6
1
7
9

the empty cell is highlighted in yellow

numbers in row 6

123456789

missing: 6

numbers in column 9

123456789

missing: 6

numbers in the box

123456789

missing:

answer: the cell must be 6because it's the only number missing from row 6, column 9, and the box!

example 2: step-by-step elimination

let's work through the logic step by step. we'll check each constraint.

step 1: check the row

5
3
4
6
7
8
9
1
2
6
7
2
1
9
5
3
4
8
1
9
8
3
4
2
5
6
7
8
5
9
7
6
1
4
2
3
4
2
6
8
5
3
7
9
1
7
1
3
9
2
4
8
5
9
6
1
5
3
7
2
8
4
2
8
7
4
1
9
6
3
5
3
4
5
2
8
6
1
7
9

row 6 has: 1, 2, 3, 4, 5, 7, 8, 9
missing: 6

step 2: check the column

5
3
4
6
7
8
9
1
2
6
7
2
1
9
5
3
4
8
1
9
8
3
4
2
5
6
7
8
5
9
7
6
1
4
2
3
4
2
6
8
5
3
7
9
1
7
1
3
9
2
4
8
5
9
6
1
5
3
7
2
8
4
2
8
7
4
1
9
6
3
5
3
4
5
2
8
6
1
7
9

column 9 has: 1, 2, 3, 4, 5, 7, 8, 9
missing: 6

step 3: check the box

5
3
4
6
7
8
9
1
2
6
7
2
1
9
5
3
4
8
1
9
8
3
4
2
5
6
7
8
5
9
7
6
1
4
2
3
4
2
6
8
5
3
7
9
1
7
1
3
9
2
4
8
5
9
6
1
5
3
7
2
8
4
2
8
7
4
1
9
6
3
5
3
4
5
2
8
6
1
7
9

the box has: 1, 2, 3, 4, 5, 7, 8, 9
missing: 6

6

all checks pass!

6 is the only number that satisfies all three constraints.

example 3: a different position

this time, the empty cell is in a different location. the same logic applies!

3
4
6
7
8
9
1
2
6
7
2
1
9
5
3
4
8
1
9
8
3
4
2
5
6
7
8
5
9
7
6
1
4
2
3
4
2
6
8
5
3
7
9
1
7
1
3
9
2
4
8
5
6
9
6
1
5
3
7
2
8
4
2
8
7
4
1
9
6
3
5
3
4
5
2
8
6
1
7
9

numbers in row 1

123456789

numbers in column 1

123456789

numbers in the box

123456789

row 1 missing: 5
column 1 missing: 5
box 1 missing: 5
→ answer is 5!

summary

1

look at the row

what numbers are already there?

2

look at the column

what numbers are already there?

3

look at the box

what numbers are already there?

the missing number — the one that's not in any of the three — is your answer!

← back to tutorialnext: obvious single →