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"
[http://docs.sun.com/source/806-3568/ncg_goldberg.html]
explores this in detail.