The Pareto Principle—sometimes known as the 80/20 rule—states that for many outcomes, roughly 80% of consequences come from 20% of causes (the “vital few”). It can be applied to economics (80% of the wealth is held by 20% of the people), software engineering (fix 20% of the bugs, eliminate 80% of the crashes), and a host of other domains.
In jigsaw puzzles, 80% of the frustration fun will derive from the remaining 20% of the pieces to place.