fifteen

Fifteen is the integer that comes after fourteen and before sixteen. In decimal notation it is written as 15. In other common bases it appears as 1111 in binary, 17 in octal, and F in hexadecimal. In Roman numerals it is XV.

Mathematically, 15 is a composite number with prime factorization 3 × 5. Its divisors are 1, 3, 5, and 15. It is a triangular number, equal to the sum of the first five natural numbers (1 + 2 + 3 + 4 + 5). It is also a hexagonal number (the third hexagonal number, since H3 = 3 × (2 × 3 − 1) = 15). Additionally, 15 is one less than 16 and can be expressed as 4 squared minus 1.

Etymology and representation: the English word fifteen derives from Old English fiftēne, combining elements related to five and ten, with the -teen suffix denoting the ten. The Latin form quindecim is the ancestor of cognates in many languages (for example, French quinze, Spanish quince, Italian quindici). The name reflects a historical pattern of forming teens as a value tied to ten.

In culture and usage, fifteen often appears in timekeeping and age-related references: fifteen minutes make a quarter of an hour, and many cultures mark coming of age at fifteen through different traditions, such as the quinceañera in Latin American communities. In sports, a standard rugby union team fields fifteen players on the field.