21

Number
$21$ (twenty-one) is:


 * $3 \times 7$


 * The $6$th triangular number, after $1, 3, 6, 10, 15$:
 * $21 = 1 + 2 + 3 + 4 + 5 + 6 = \dfrac {6 \times \left({6 + 1}\right)} 2$


 * Thus $21$ is the number of pips on a die.


 * The smallest number which can be expressed as the sum of at most $3$ triangular numbers in $4$ ways:
 * $21 = 15 + 6 = 15 + 3 + 3 = 10 + 10 + 1$

Also see

 * Smallest Number Expressible as Sum of at most Three Triangular Numbers in 4 ways
 * Smallest Perfect Square Dissection