The answer is 40.
Why? 1+4 =5, 2+5 (add the first line 1+4) that is =12 3+6 (add both the above lines) that is =21 8+11 (add all the other three lines) that is =40.
Mathematical puzzles make up an integral part of recreational mathematics. They have specific rules as do multiplayer video games, but they do not usually involve competition between two or more players. Instead, to solve such a puzzle, the solver must find a solution that satisfies the given conditions. Mathematical puzzles require mathematics to solve them. Logic puzzles are a common type of mathematical puzzle
Conway’s Game of Life and fractals, as two examples, may also be considered mathematical puzzles even though the solver interacts with them only at the beginning by providing a set of initial conditions. After these conditions are set, the rules of the puzzle determine all subsequent changes and moves. Many of the puzzles are well known because they were discussed by Martin Gardner in his “Mathematical Games” column in Scientific American. Mathematical puzzles are sometimes used to motivate students in teaching elementary school math problem solving techniques. Creating thinking (Thinking outside the box) often helps to find the solution.
To know more about puzzles click here