arithmetic

Computer arithmetic is not the same as grade-school arithmetic; for example 0.1 + 0.1 is unlikely to be 0.2.

See:

1. Fixed-point arithmetic
2. Decimal arithmetic
3. Decimal Arithmetic Package for tcl 8.5
4. TCL command: expr

Also:

1. Goldberg, David; "What Every Computer Scientist Should Know About Floating-Point Arithmetic" (1991) ; which explores this in detail.