Use the following applet to explore the basics in modular (or "clock") arithmetic.
Consider: On a clock, there are 12 hours...what do we call hour 13? Similarly, there are 60 minutes, and 60 seconds on a clock...but what do we call minute/second 61? Suppose instead of having 12 hours on a clock, we had only 7. What would we call hour 11? Can you write an equation showing the relationship between a, b and n?