Grid Hack ($0.99) by KST Net-tech,ltd is a puzzle game based on Sudoku, but with an extra challenge. You have a grid of squares which must be filled up with numbers in such a way that each row and column has each number appear only once. The numbers must also give the correct answer to a math problem.

In Sudoku, you have a 9×9 grid, where the numbers one to nine are used to filled the empty squares. For Grid Hack, the grid can be anything from a 3×3 grid up to the usual 9×9 grid.

Each size has nine puzzles to complete under a time trial. You start off with five stars and the longer you take to solve the puzzle, the more stars you lose. Once you have earned at least 35 stars out of a maximum of 45, you unlock the next size grid.

This makes getting to the full nine by nine games take a little while, but there’s an extra element which makes it challenging even on the smaller grids.

Some of the squares will be grouped together, indicated by being made the same color. These groups will have an additional requirement given by a small number and a math operator on their top left. The requirement tells you what the numbers on the colored squares should be doing.

For example, if you have two yellow squares with a “two minus” written on the top left, that means that the two numbers you choose to fill the two yellow squares must, when the larger number is taken away from the smaller, be able to give the answer of minus two (-2).

The order you write the numbers does not matter as far as the sum is concerned, but you will still have to make sure you satisfy the overarching aim which is to have one of each number in each row and column.

Similarly, if you have four blue squares with “four times” written on the top left, the numbers you choose to fill the four blue squares must, when multiplied, equal four.

This adds a great new degree of depth to the game which will appeal to those who love both math and puzzles. I can well imagine that he 9×9 grids are very challenging indeed. Grid Hack is free so don’t hesitate to download it.