80

Number
$80$ (eighty) is:


 * $2 \times 4 \times 5$


 * The $18$th semiperfect number after $6$, $12$, $18$, $20$, $24$, $28$, $30$, $36$, $40$, $42$, $48$, $54$, $56$, $60$, $66$, $72$, $78$:
 * $80 = 2 + 8 + 10 + 20 + 40$


 * The $42$nd positive integer after $2$, $3$, $4$, $7$, $8$, $\ldots$, $50$, $54$, $55$, $59$, $60$, $61$, $65$, $66$, $67$, $72$, $77$ which cannot be expressed as the sum of distinct pentagonal numbers.