15
Number 15 The Meaning of Number 15 Fun Facts. Number 15 is a triangular number: 1+2+3+4+5=15 Phosphorus has the atomic number of 15 A crystal anniversary celebrates fifteen years May 15th is the International Day of Families February 15th is Susan B Anthony Day celebrated in the US Susan B Anthony (February 15, 1820 March 13, 1906) worked all her life for the rights of women