680

Number
$680$ (six hundred and eighty) is:
 * $2^3 \times 5 \times 17$


 * The $2$nd tetrahedral number after $20$ which is the sum of two tetrahedral numbers:
 * $680 = 120 + 560$


 * The $15$th tetrahedral number, after $1$, $4$, $10$, $20$, $35$, $56$, $84$, $120$, $165$, $220$, $286$, $364$, $455$, $560$:
 * $680 = \ds \sum_{k \mathop = 1}^{15} \frac {k \paren {k + 1} } 2 = \dfrac {15 \paren {15 + 1} \paren {15 + 2} } 6$