Thanos has invaded the Avengers. In the final standoff, Iron Man is off to fight the Mad Titan. Iron Man has created anti-infinity Stones to finish Thanos. But, the anti-Infinity stones can only be used together using a password. Iron Man has created a password of 6 numbers, where each number represents an anti-Infinity Stone.

 Iron Man only remembers 5 and has forgotten the 6th. What should be the 6th number so that the billionaire genius can destroy Thanos?

  The first 5 numbers of the password are:






What should be the final number for the password? (Iron Man needs to quickly remember the 6th number, before Thanos figures out the logic behind the password!!!)

  • 4
    $\begingroup$ ...but it was too late... and the final words heard from humankind escaped the mouth of the Herculean being by the name of Thanos: "I am inevitable." SNAP xD $\endgroup$ – Mr Pie Apr 28 '19 at 21:37
  • 16
    $\begingroup$ I just hope that nobody hides an End Game spoiler in an answer's spoiler $\endgroup$ – Daniel Duque Apr 28 '19 at 23:54
  • 3
    $\begingroup$ Spoiler: rot13(uggcf://vzntrf-an.ffy-vzntrf-nznmba.pbz/vzntrf/V/614VCBh86dY._FL355_.wct) $\endgroup$ – Vilx- Apr 29 '19 at 8:25

Gareth's reasoning is correct. However,

what he gave was the 7th number. There is another smaller number that fulfils the condition, and would thus be next in sequence: 521001000

  • $\begingroup$ Ooops! I confess I assumed from the framing story that there were exactly six such numbers, so when I found another I assumed it was the next one. Good catch! $\endgroup$ – Gareth McCaughan Apr 29 '19 at 12:35

The sixth number is


and these are

numbers that count their own digits. So, e.g., 1210 has one 0, two 1s, one 2, zero threes.

  • 3
    $\begingroup$ This is OEIS sequence A138480 $\endgroup$ – Chronocidal Apr 29 '19 at 9:45
  • $\begingroup$ Close call. You almost had it much like the Gauntlet from Thanos in Infinty War!! $\endgroup$ – chandrodoy banerjee Apr 29 '19 at 12:34
  • $\begingroup$ D'oh: if I'd thought to check OEIS I wouldn't have missed the fact that there's another smaller one :-). $\endgroup$ – Gareth McCaughan Apr 29 '19 at 12:35

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