Balls in a box

Author:
Ben Sparks
What are the chances of pulling two balls of the same colour from a bag containing white balls and black balls? If you'd like the probability to be exactly , which amounts of white and black could you use? (source - Jonny Griffiths - Making Stats Vital - http://www.s253053503.websitehome.co.uk/msv/msv-23.html)