Last month, British student Alex Chui became the all-time most successful contestant at the International Mathematical Olympiad. No other contestant in the history of the IMO, which has been running since 1959 and brings together school-age contestants from more than a hundred countries, has won a medal seven years in a row. I asked Alex if he had a favourite puzzle, and he suggested the following gem. When you multiply the three numbers in each row, and in every column, the answer is 30. The three numbers in each row or column do not have to be three different numbers.]