80 (number)

← 79 80 81 →
← 80 81 82 83 84 85 86 87 88 89 → List of numbers Integers ← 0 10 20 30 40 50 60 70 80 90 →
Cardinal	eighty
Ordinal	80th (eightieth)
Factorization	24 × 5
Divisors	1, 2, 4, 5, 8, 10, 16, 20, 40, 80
Greek numeral	Π´
Roman numeral	LXXX, lxxx
Binary	10100002
Ternary	22223
Senary	2126
Octal	1208
Duodecimal	6812
Hexadecimal	5016
Armenian	Ձ
Hebrew	פ / ף
Babylonian numeral	𒐕⟪
Egyptian hieroglyph	𓎍

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.
• 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.
• the sum of the first 4 twin prime pairs ((3 + 5) + (5 + 7) + (11 + 13) + (17 + 19)).

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 science

Element 80: Mercury (Hg)

• The atomic number of mercury

In religion

• 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.

See also

• List of highways numbered 80

External links

• wiktionary:eighty for 80 in other languages.