15 (number)

15 is a natural number that falls between 14 and 16 in sequence. It is written as '15' in decimal notation and as 'XV' in Roman numerals.

15 is a composite number, meaning it has divisors other than 1 and itself. Its divisors are 1, 3, 5, and 15. Its prime factorization is 3 × 5, making it a semiprime—a product of exactly two prime numbers.

Mathematical properties

15 is a triangular number: it is the sum 1+2+3+4+5. It is also a hexagonal number. The sum of its proper divisors (1+3+5) is 9, which is less than 15, so 15 is a deficient number.

In the 3×3 magic square, all rows, columns, and both main diagonals sum to 15. This is a notable reason why 15 features prominently in the study of magic squares.

Representation in other bases

In binary, 15 is represented as 1111—four bits all set to one. For this reason, in hexadecimal it is represented by the letter 'F', the highest single-digit value. This makes 15 an important number in computing and programming.

Common usage

In everyday life, 15 denotes a quarter of an hour (15 minutes) and 15 percent. In rugby union, each team has 15 players on the field. In tennis, the first point in a set is called '15'. Moreover, 15 years marks a culturally and legally significant milestone in many societies, notably the quinceañera in Latin American tradition.