Number reference
15 explained.
15: fifteen, composite, triangular number; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- fifteen
- Classification
- composite
- Factors
- 1, 3, 5, 15
- Prime factorization
- 3 × 5
- Square root
- 3.872983
- Cube root
- 2.466212
- Binary
- 1111
- Octal
- 17
- Hexadecimal
- F
- Roman numeral
- XV
- Digit sum
- 6
- Digital root
- 6
What makes 15 notable
- Triangular number. 15 is the 5th triangular number: it is the sum 1 + 2 + … + 5, the count of items in a triangle with 5 rows.
15 in everyday use
Fifteen is the atomic number of phosphorus, the number of players on a rugby union side, and the first point scored in a tennis game. A quinceañera marks a girl's fifteenth birthday across Latin America. Julius Caesar was assassinated on the Ides of March, the fifteenth. A quarter of an hour is fifteen minutes, and the sliding 15-puzzle, with fifteen numbered tiles in a four-by-four frame, was a craze in 1880 long before the Rubik's Cube.
Fifteen is the fifth triangular number, 1 + 2 + 3 + 4 + 5, which is why fifteen balls rack into a triangle on a pool table. It is the magic constant of the 3 × 3 magic square: every row, column, and diagonal of the digits 1 to 9 arranged correctly sums to 15. It is the product of the first two odd primes, 3 × 5, and in hexadecimal it is F, the largest single digit.
Nearby numbers
How to read this page
The factor list shows every way 15 splits into equal whole-number groups — 4 divisors in total. The prime factorisation is the compact form of that same information, and the base conversions restate 15 for contexts that do not use decimal.
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