A firm understanding of solving for the unknown, the foundation behind algebra, is required to solve missing number puzzles. You need to recognize the patterns or rules of the information you have and apply it to the unknown variable.
Analyze the information you already have
Every missing number puzzle will provide information that surrounds or precedes the missing value. This information might be provided in written hints or rule, but it's most often in the form of an arrangement of numbers. If there are listed rules to the puzzle, read the rules and check your understanding by examining how the given information follows the rules. For example, if you are starting a Sudoku puzzle, you are given a grid in which some numbers have been filled in. The rules state that each 3-by-3 box must have one of each digit 1-9 and that the digits don't repeat within any row or column.
Determine the pattern and test for consistency
If the pattern isn't provided in the written rules, you must find the pattern. If the puzzle has a list of increasing values, determine the rate of increase. If the puzzle involves clusters of associated numbers, see what relationships exist between the numbers. Once you have an idea of a rule, test it on known values.
Use the rule you found to solve for the missing number
Set up an equation that uses your rule. Plug in any of the given values that the unknown corresponds to, and write the unknown as a variable. Isolate the variable, and solve for it.