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] 1. [Decimal arithmetic] 1. [Decimal Arithmetic Package for tcl 8.5] 1. TCL command: [expr] Also: 1. [http://docs.sun.com/source/806-3568/ncg_goldberg.html%|%Goldberg, David; "What Every Computer Scientist Should Know About Floating-Point Arithmetic" (1991)%|%]; which explores this in detail. <> Mathematics