Question 21
Leonhard is designing a puzzle for Katharina. It has nine squares in a 3 × 3 grid and a number of clues. Each clue is a number 1, 2 or 3 placed in one of the squares. Katharina then has to find a solution by placing 1, 2 or 3 in each remaining square so that no row or column has a repeated number. What is the smallest number of clues that Leonhard could include so that his puzzle has exactly one solution?
