Clock arithmetic and modular arithmetic
http://www.shodor.org/interactivate/discussions/morerem.html
This site gives a nice introduction to modular arithmetic.
http://betterexplained.com/articles/fun-with-modular-arithmetic/
This site is a nice interactive site which deals with arithmetic (mod m). Do not be frightened by the notation, the arithmetic table at the bottom is interesting
http://www.cut-the-knot.com/blue/Modulo.shtml
Is there really such a thing as “infinity”?
http://www.math.toronto.edu/mathnet/plain/answers/infinity.html