Computer arithmetic is not the same as grade-school arithmetic; "Fixed-point arithmetic", for example, mentions that, in the former, 0.1 + 0.1 is unlikely to be 0.2. "What Every Computer Scientist Should Know About Floating-Point Arithmetic" [L1 ] explores this in detail.
[Explain expr's history.]
See also Decimal arithmetic, [(back?)-references to other pertinent pages]