820

Number
$820$ (eight hundred and twenty) is:


 * $2^2 \times 5 \times 41$


 * The $40$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $325$, $351$, $378$, $406$, $435$, $465$, $496$, $528$, $561$, $595$, $630$, $666$, $703$, $741$, $780$:
 * $820 = \displaystyle \sum_{k \mathop = 1}^{40} k = \dfrac {40 \times \left({40 + 1}\right)} 2$