The crossword solution for the term 'When Doubled, A Popular Number Puzzle' is KEN, which is a three letter word. KenKen is a popular number puzzle that was created by Japanese math teacher Tetsuya Miyamoto in 2004. It is similar to a Sudoku puzzle, but with the added challenge of having to use basic math operations to solve it. The goal of the game is to fill in the grid with numbers so that each row and column contains the numbers 1-9 without repeating any numbers.