20

Twenty is the natural number that follows nineteen and precedes twenty-one. In decimal notation it is written as 20. In other common numeral systems it appears as binary 10100, octal 24, and hexadecimal 14, while in Roman numerals it is XX.

The word twenty derives from Old English twentig, from Proto-Germanic roots meaning “two tens.” It is related to other Germanic forms such as German zwanzig.

In mathematics, twenty is an even composite number. Its prime factorization is 2^2 × 5, and its divisors are 1, 2, 4, 5, 10, and 20 (six divisors in total). The sum of its proper divisors is 22, making 20 an abundant number. Its Euler totient value is φ(20) = 8. Twenty is also a semiperfect number, since 10 + 5 + 4 + 1 = 20 uses distinct proper divisors.

In science, calcium has atomic number 20 and symbol Ca. Calcium is the fifth most abundant element by weight in the Earth’s crust and is essential for bone and teeth formation, muscle function, nerve signaling, and blood clotting.

Culturally, twenty has several common associations. The term score refers to twenty; the phrase four score and seven years uses this value. The 20th anniversary is traditionally associated with china, with platinum listed as a modern alternative. Vision is described as 20/20, indicating standard visual acuity.