20 (number)

20 (twenty) is a natural number that comes after 19 and before 21. In the decimal (base-10) number system, it is composed of two digits: 2 and 0, meaning two tens.

It is an even number, as it can be divided by two with no remainder. It is not a prime number; its divisors are 1, 2, 4, 5, 10, and 20. It is the product of prime factors: 2 × 2 × 5, or 2² × 5.

Mathematical properties

The sum of its proper divisors (those less than 20: 1+2+4+5+10) is 22, which exceeds the number itself. For this reason, 20 is classified as an abundant number. It is also a tetrahedral number—the sum of the first four triangular numbers (1+3+6+10 = 20).

Number systems

Many cultures made 20 the basis of a vigesimal (base-20) counting system, based on the total number of fingers and toes. A well-known example is the Maya system and certain European languages, such as French, in which 80 is called 'quatre-vingts' (four twenties).

Somali language and usage

In Somali, 20 is called 'labaatan', derived from 'laba' (two) and toban (ten); thirty, forty, and others follow the same pattern. Labaatan is commonly used in everyday measurements, such as divisions of time, money, and counting.

Other contexts

20 appears in many non-mathematical contexts: the scoring system in darts, the twenty faces of an icosahedron (a geometric solid), and the number of standard amino acids from which proteins are built.