The 24th post reference was a way of announcing having reached 11000 posts, it being the binary equivalent of 24.
Regarding Jooels comment on the example: The correct number is 123456789. Therefore one would have to change the second digit to a "2". Any other changes (in addition to the changes to "2"), e.g.changing the 5 to a 6, would violate the “only one garbled digit” criterion.
Regarding the positons of the digits, for example check number 123456789 has a checksum of 0:
1 2 3 4 5 6 7 8 9
d9 d8 d7 d6 d5 d4 d3 d2 d1
Hope that clears things up.
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There are two types of vessels, submarines and targets.
