80 (eighty) is the natural number following 79 and preceding 81.
80 is:
the sum of Euler's totient function φ(x) over the first sixteen integers.
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 (22223), 6 (2126), 9 (889), 15 (5515), 19 (4419) and 39 (2239).
a repdigit in bases 3, 9, 15, 19 and 39.
a Harshad number in bases 2, 3, 4, 5, 6, 7, 9, 10, 11, 13, 15 and 16
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 Fifteen puzzle can be solved in no more than 80 single-tile moves.
The atomic number of mercury
According to Exodus 7:7, Moses was 80 years old when he initially spoke to Pharaoh on behalf of his people. Today, 80 years of age is the upper age limit for cardinals to vote in papal elections.
Jerry Rice wore the number 80 for the majority of his NFL career. The number 80 is also the retired number of Seattle Seahawks wide receiver Steve Largent; when Rice was traded to the Seahawks, Largent gave the team permission to unretire his number so Rice could wear it.
Eighty is also:
used in the classic book title Around the World in Eighty Days
the length of the Eighty Years' War or Dutch revolt (1568–1648)
the standard TCP/IP port number for HTTP connections
the 80A, 80B and 80C photographic filters correct for excessive redness under tungsten lighting
The year AD 80, 80 BC, or 1980
Eighty shilling ale
I-80 is an interstate highway going from New York to San Francisco
The older four-pin-base version of the 5Y3GT rectifier tube
A common limit for the characters per line, in computing, derived from the number of columns in IBM cards
American band Green Day has a song called "80"
A fictional alien superhero named Ultraman 80
