puzzles can be adjusted in several ways. One of the simplest method is to set a ratio of the number of the clue cells to the total number of the cells on the board. However, this may result a largely varying complexity range for a fixed ratio. Another method is to reduce clue cells based on some problem solving strategies step by step. The complex strategies may be enabled for high complexity levels such as subtracting an equation with another one, or the higher depth of try-and-check steps. When the board size increases, the range of the problem cases increases. The ratio of the number of hidden objects to the total number of cells affects the complexity of the puzzle too. Notes References Paul Halmos, Naive set theory. Princeton, NJ: D. Van Nostrand Company, 1960. Reprinted by Springer-Verlag, New York, 1974. (Springer-Verlag edition). Alexander Schrijver, Theory of Linear and Integer Programming. John Wiley & Sons, 1986. Reprinted in 1999. . Adam Drozdek, Data Structures and Algorithms in C++, Brooks/Cole, second edition, 2000. . Puzzles Algebra Variables (mathematics) Equations