The first calculations we learn at school are related to the natural numbers (1,2,3,….), the we learn rational numbers (fractions), then negative and irrational numbers are introduced to fill the real numbers. Theoretical Mathematics is based on exact representation of numbers, and that mathematical operations are exact. When using computers to perform calculations most often floating point numbers are used to simplify the representation. The consequence is that instead of using all real numbers, one a very limited set of numbers is used. An operation between two such floating point numbers will often not be possible to represent exactly as a floating point number. Within approximation theory we find methods and for how to best describe and solve mathematical problems on computers.