80 (number)

80 (eighty) is the natural number following 79 and preceding 81.

← 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

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.^[1]
• a ménage number.^[2]
• 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.^[3][4]

Every solvable configuration of the 15 puzzle can be solved in no more than 80 single-tile moves.^[5]