Adding and taking away within 18 · Usually met around P1
4 × 4 sudoku
Cross out what its row, column and box have.
How to do it
Method
Cross out what its row, column and box have
The row has 1, 2, 4 → 3 is missing
Another way
They add up to 10
10 − 1 − 2 − 4 = 3
In full
In a sudoku every row, every column and every bold box (the 2 × 2 squares) must have 1, 2, 3 and 4 exactly once. First look for a row, column or box with one number missing: see which of 1 to 4 isn't there, and write it. Then take an empty square: cross out the numbers already in its row, its column and its box; the one left is the answer. Every square you fill gives you new clues. Or add: the four numbers in a row always add up to 10.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
Every row, column and bold box of a sudoku must have 1, 2, 3 and 4. What goes where the ? is?
Show the answer and working
Answer: 1
The ?'s row already has 4, 2 and 3; of 1 to 4, only 1 is missing.
Example 2
Every row, column and bold box of a sudoku must have 1, 2, 3 and 4. What goes where the ? is?
Show the answer and working
Answer: 2
Its row and column don't say enough, but its bold box already has 3, 1 and 4: only 2 is missing.
Example 3
Every row, column and bold box of a sudoku must have 1, 2, 3 and 4. Which number goes where the ? is?
- A. 3
- B. 1
- C. 2
- D. 4
Choose the right answer
Show the answer and working
Answer: D. 4
The row has 3 and 2, so the ? is 4 or 1; the column has 1, so it is 4.
Practise it in the game →Adding and taking away within 18 · Free to start, unlimited questions, no download