I learned something new today, that the 80/20 rule comes from Italian economist and engineer Vilfredo Pareto (Wiki link)
The story starts with the observation that 20% of the pea pods in the garden, produce 80% of the peas. Pareto became curious and looked at other data. He found that 80% of the land was owned by 20% of the population in Italy. The Wikipedia link goes into the equations for the Pareto index.
Many have heard of the 80/20 rule, but many probably didn't know its origin. It can be used as a rule of thumb for so many estimates, many human endeavors. I have heard it said that in most volunteer organizations, 80% of the work is done by 20% of the volunteers. It might often be more skewed than that. Some use the 80/20 rule in making business decisions. This might result in focusing on the most productive 20% of employees, or 20% most valuable of customers.