http://www.moserware.com/2009/09/stick-figure-guide-to-advanced.html

How does Modular Arithmetic work?

https://www.youtube.com/watch?v=5OjZWSdxlU0

How To Find The Inverse of a Number ( mod n ) - Inverses of Modular Arithmetic - Example

https://www.youtube.com/watch?v=shaQZg8bqUM

Modular arithmetic with Fermat and Euler | Data Structures in Mathematics Math Foundations 197

https://www.youtube.com/watch?v=9DeOnCKfSuY

Division in modular arithmetic is done by multiplying the number that you want to divide by the inverse of the dividend. An example is that 8/2 is also equal to 8 * (1/2).

It is good to watch these, since modular arithmetic used in AES and Elliptical Curve Cryptography.