Fifteen (15) is a positive integer one more than 14 and one less than 16.

## PropertiesEdit

15 is the second odd composite number, being 3*5. It is also a semiprime.

Because 15's factorization only contains Fermat primes repeated once, a regular 15-sided polygon is constructible with compass and straightedge.

15 is the fifth triangular number (between 10 and 21), and the third pentatope number (between 5 and 35).

15 is a lucky number, between 13 and 21.

15 is equal to 5!!, the double factorial of 5.

In Czech language, 15 is called "mandel".

## In googology Edit

In Greek-based number-naming systems, 15 is associated with prefix "**pentadeka-**", and with prefix "**quindec-**" in Latin systems.

Sbiis Saibian calls the number **pentaxis**, and it's equal to 10+5 using the addition

## As a banknote denomination Edit

The third Template:W had banknotes with this number in the denomination.