FlipFour

: FlipFour is a simple math puzzle. A set of white and black cells with numbers are given. When flipped, a white number will turn to black and vice versa. You must flip four cells to get a given sum for all white numbers. Each puzzle has a unique solution. Puzzles come in 9 different sizes. This puzzle appears deceptively easy but require considerable mental maths to solve. This is how you solve a puzzle on screen:1. Examine the numbers displayed, with white or black background2. Now consider how to get a required sum of white numbers3. Certain whites may have to be flipped to black (for not to be included in the sum) or some blacks should be whites (to get added to the sum)4. Then click on a cell to flip, a black number will flip to white and vice versa. Sum of the white numbers will be updated accordingly.5. You must flip four different cells (no less no more). For example if in a puzzle, the current and the required sum of white cells is same, user cannot click a cell four times to solve.6. Click on any place outside the Grid to undo a flip7. Running total is given at the bottom right of the window8. You can also use arrow keys to get to a cell and press Spacebar or Return to flip the colour. Press Delete undo the previous flip.9. Puzzle is solved when after four flips all the white numbers add up to the required sum.