In Cryptarithmetic puzzles, we are given an arithmetic problem using letters instead of numbers. Each letter represents a single digit: 0 to 9. The goal is to determine which letter corresponds to which digit, so that the arithmetic problem is correct. Ideally, these have only one solution, and it is possible to find that solution without guessing.
For a nice collection of these puzzles, try Sideways Arithmetic From Wayside School by Louis Sachar.