forty

Forty is the natural number that comes after thirty-nine and before forty-one. It is an even, composite integer with prime factorization 2^3 × 5 and eight positive divisors: 1, 2, 4, 5, 8, 10, 20, and 40. In common numeral systems it is 40 in decimal, 101000 in binary, 50 in octal, and 28 in hexadecimal; in Roman numerals it is written XL.

The name forty derives from four plus the suffix -ty, from Old English feortig and Proto-Germanic *faurtīg, indicating four tens. It is commonly noted that the English word "forty" is the only number whose letters are in alphabetical order.

Mathematically, 40 is divisible by the sum of its digits (4 + 0 = 4), making it a Harshad number in base 10. It is the product of two integers 5 and 8 and has eight divisors in total.

In culture and daily life, forty appears in various idioms and traditions: forty days and forty nights; the tale of Ali Baba and the Forty Thieves; and the biblical or liturgical use of forty days of Lent. In modern contexts, a standard workweek is often described as about forty hours. In sports, tennis uses the score of 40 as the third point in a game. In science, atomic number 40 corresponds to zirconium (Zr).