> 80/20 rule: 20% of customers were responsible for 80% of revenues

> Sometimes you will see a 90/10 relationship or a 75/25 split.

It's funny that his examples add up to a nice round 100, but it doesn't have to. The number of customers and percentage of revenue are two independent measures, and they don't have to sum to 100. You could have 20% of customers giving 99% of revenue (a 99/20 relationship) or 10% of customers giving 60% of revenue (a 60/10 split). I assume the author is well aware of this, but likes his examples to sound like the well-known 80/20.