Normal Sudoku Rules. The sum of the digits along the path of each arrow must equal the digit in the circled cell (digits can repeat on an arrow). Along thermometers, digits must increase from the bulb end. The sum or product of the digits in each cage must be either 20 or 21. The sum or product of the digits along the marked diagonals outside the grid must be either 20 or 21 (digits can repeat on a diagonal).