2
$\begingroup$

1, 3, 6, ?, ?, ?, 45, 55

I asked the person who gave this to me and they assured me that it's correct.

After an hour, I've given up :(

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4
$\begingroup$

I think the answer is

1, 3, 6, 15, 28, 36, 45, 55

Because

These are the triangular numbers whose digits are in non-decreasing order.

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  • $\begingroup$ Where are 10 and 21? $\endgroup$ – iminiki Feb 6 at 12:50
  • 2
    $\begingroup$ Their digits are not in nondecreasing order. $\endgroup$ – Gareth McCaughan Feb 6 at 12:52

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