80 (eighty) is the natural number following 79 and preceding 81.
In mathematics
80 is:
- The sum of Euler's totient function φ(x) over the first sixteen integers:
: 1 + 1 + 2 + 2 + 4 + 2 + 6 + 4 + 6 + 4 + 10 + 4 + 12 + 6 + 8 + 8 = 80.
- A semiperfect number, since adding up some subsets of its divisors (e.g., 1, 4, 5, 10, 20, and 40) gives 80.
- A ménage number.
- Palindromic in bases 3 (2222<sub>3</sub>), 6 (212<sub>6</sub>), 9 (88<sub>9</sub>), 15 (55<sub>15</sub>), 19 (44<sub>19</sub>), and 39 (22<sub>39</sub>).
- A repdigit in bases 3, 9, 15, 19, and 39.
- The sum of the first four twin prime pairs:
: (3 + 5) + (5 + 7) + (11 + 13) + (17 + 19) = 80.
The Pareto principle (also known as the 80-20 rule) states that, for many events, roughly 80% of the effects come from 20% of the causes.
Every solvable configuration of the 15 puzzle can be solved in no more than 80 single-tile moves.
In trade
In Northern Europe, there have traditionally been words for eighty units - often eggs or herrings: ' in German, ol in Danish, and øll in Norwegian (all cognates).