405

Number
$405$ (four hundred and five) is:


 * $3^4 \times 5$


 * The $9$th pentagonal pyramidal number after $1, 6, 12, 40, 75, 126, 196, 288$:
 * $405 = 1 + 5 + 12 + 22 + 35 + 51 + 70 + 92 + 117 = \dfrac {9^2 \left({9 + 1}\right)} 2$