Our culture has a base 10 system and uses digits and place holders. We have been exposed to this system our whole lives and find it hard to imagine any other way. The truth is that our system is based on choices we have made rather than by some inherent quality of numbers or math. Different cultures have invented math and numbers in different ways. Take a look at the link below to get a sense of the Mayan system which was a base 20 system.
http://en.wikipedia.org/wiki/Maya_numerals
Here is a handout I made based on this topic:
TOK Topics 