Twenty is the number that is after nineteen and before twenty-one.
The prime factors of twenty are 2 and 5. (2 * 2 * 5 = 20)
Its factors are: 1, 2, 4, 5, 10, and 20. As the sum of its factors is more than itself (i.e. 22), it can be referred to as an abundant number.
20 can been used as a number base. Remnants of this system remain in some European languages, for example in the English “score” (20) and the French “quatre-vingts” (80, literally four groups of twenty). The old (pre-decimal) English monetary system had twenty shillings in a pound. The ancient Mayan numerical system – counting on fingers and toes – was a base 20 or "vigesimal" system.
A polyhedron of 20 faces is an icosahedron, one of the five Platonic solids. It is a convex regular polyhedron composed of twenty triangular faces, with five meeting at each of the twelve vertices. It has 30 edges and 12 vertices. Its dual polyhedron is the dodecahedron.
In Japanese tradition, adulthood is established at the age of 20. See seijin not hi (the celebration of adulthood in Japan).
It is the number of milk teeth in an infant’s mouth.
In Roman numerals, 20 is written as XX.