280

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Number

$280$ (two hundred and eighty) is:

$2^3 \times 5 \times 7$

The $10$th octagonal number, after $1$, $8$, $21$, $40$, $65$, $96$, $133$, $176$, $225$:

$280 = 1 + 7 + 13 + 19 + 25 + 31 + 37 + 43 + 49 + 55 = 10 \paren {3 \times 10 - 2}$

The $42$nd happy number after $1$, $7$, $10$, $13$, $19$, $23$, $\ldots$, $188$, $190$, $192$, $193$, $203$, $208$, $219$, $226$, $230$, $236$, $239$, $262$, $263$:

$280 \to 2^2 + 8^2 + 0^2 = 4 + 64 + 0 = 68 \to 6^2 + 8^2 = 36 + 64 = 100 \to 1^2 + 0^2 + 0^2 = 1$

Also see

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