# 24 Little-Known Facts About the Number 15

Facts About Number 15: Welcome to our list of fascinating facts about the number 15! Did you know that 15 is the number of days in a fortnight, the number of years it takes for Saturn to orbit the Sun, and the number of keys on a traditional piano? In this article, we'll explore 15 interesting and surprising facts about the number 15, from its role in math and science to its cultural significance and historical significance. Whether you're a math enthusiast or just curious about the world around you, we hope you'll enjoy learning about this versatile and important number.

• The number 15 is a composite number, meaning that it can be expressed as the product of two smaller integers.
• 15 is the smallest composite number that is odd and not a prime.
• The Roman numeral for 15 is XV.
• 15 is the sum of the first four prime numbers (2 + 3 + 5 + 7).
• 15 is the third triangular number, meaning that it is the number of dots in a triangle with three dots on each side.
• The atomic number of phosphorus is 15.
• In base 10, the number 15 has the digits "1" and "5", which add up to 6.
• In the date format commonly used in the United States, "15" can represent the 15th day of any month.
• 15 is a Harshad number in base 10, meaning that it is an integer divisible by the sum of its digits.
• The 15th element in the periodic table is phosphorus.
• There are 15 cards in a suit in a standard deck of playing cards.
• The magic constant of a 3x3 magic square is 15. A magic square is a square grid of numbers where the numbers in each row, column, and diagonal add up to the same total.
• The number of days in a typical fortnight is 14, but "15 days" is sometimes used to refer to a period of time that is slightly longer than two weeks.
• 15 is the smallest number that has exactly seven positive divisors, including 1 and itself. The divisors of 15 are 1, 3, 5, and 15.
• The 15th U.S. President was James Buchanan, who served from 1857 to 1861.
• 15 degrees Celsius is equivalent to 59 degrees Fahrenheit.
• The number 15 is often used as the minimum age at which a person is allowed to obtain a driver's license in many countries.
• The 15th day of the Chinese New Year is known as the Lantern Festival, which is celebrated with lantern displays and other traditional activities.
• 15 is the number of players on a typical team in a game of Australian rules football.
• In binary, the number 15 is represented as 1111, which is the highest number that can be represented with four binary digits (bits).
• The 15th anniversary is traditionally associated with the gift of crystal.
• 15 is the smallest integer that can be written as the sum of three cubes in two different ways: 15 = 1^3 + 2^3 + 3^3 = 4^3 + 2^3 + 1^3.
• The number 15 is the smallest integer that can be written as the sum of two squares in three different ways: 15 = 3^2 + 4^2 = 4^2 + 3^2 = 6^2 + 1^2.
• 15 is the smallest number that is both a member of the Fibonacci sequence (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ...) and a Lucas number (2, 1, 3, 4, 7, 11, 18, 29, 47, 76, ...).

We hope you enjoyed learning about these 15 interesting and surprising facts about the number 15! From its role in math and science to its cultural and historical significance, the number 15 is truly a multifaceted and fascinating concept. Whether you're a math enthusiast or simply curious about the world around you, we hope you gained a greater appreciation for this important and versatile number. Thank you for joining us on this journey of discovery!