The 80/20 rule, The Pareto Principle

This rule, which dates back to 1906, says that

• 20 percent of your activities will account for 80 percent of your results,
  
• 20 percent of your products will yield 80 percent of your sales and 80 percent of your profits, and
  
• 20 percent of your customers will buy 80 percent of your products and services.

Read more about 80/20 rule at Brian Tracy's blog