On citing common knowledge and giving credit to the 'right' authors
First, the definition of "common knowledge" and I think this answer by Ben Norris from the post that Stephan Branczyk posted elucidated that pretty well (https://academia.stackexchange.com/a/2196):


  If the statement/concept/idea that you think you want to cite is covered (without citation) in the introductory undergraduate textbook(s) on the subject, then it is likely common knowledge in the field.  


So next step is to search more than one sources about that specific examples or specific numbers. If that book is the only source that came up with that example/number then you can cite from that author(s). You give credit from where you found the information. It is better to cite than not and might trouble you later on since it will give an impression that those numbers/examples are from your own findings (and which someone might cite that example from you). Because again, 'when in doubt, always cite'.