The 80/20 rule, as you may know, is a representation stating that things in the world are not evenly distributed. For instance, 80% of the wealth is distributed among 20% of the population. Or, 80% of the sales come from 20% of the customers.
One of the most popularly used examples of this is that 80% of the a company's work is done by 20% of its employees. This is actually why I'm writing on it. I'm fortunate enough to work at a place where 90% of the work is done by 90% of the staffers. In other words, people here work hard and well together.
Another thing you may not have known about the 80/20 rule is that it doesn't have to equal 100. Why? Because it measures two differing aspects. In the first example of 80% of the wealth is distributed among 20% of the population, it's measuring a) the amount of wealth and b) where it's spread among the population.
If you really want to sound smart, refer to it as the Pareto Principle. It's named after him.
Don't believe me that it doesn't have to add up to 100? Refer to this article.