550

Number
$550$ (five hundred and fifty) is:


 * $2 \times 5^2 \times 11$


 * The $10$th pentagonal pyramidal number after $1, 6, 12, 40, 75, 126, 196, 288, 405$:
 * $550 = \displaystyle \sum_{k \mathop = 1}^{10} \dfrac {k \left({3 k - 1}\right)} 2 = \dfrac {10^2 \left({10 + 1}\right)} 2$