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Grandson, when I was born, there were no less than 2,005 times... Hum. One billion people, I think. Thank you calculator! In the world.

— How come my old grandfather makes such en error?

Note 1: people who instantly gets the knowledge tag will benefit from it a lot. You are several in this website to have it for sure. I think with more research from those who don't have it, you still can figure out the puzzle and that'd be even more impressive!

Note 2: I believe it is possible that other answers apart from the one I thought exist. So I will only select the answer that matches mine, but feel free to post your tries as they'd could get +1 and a chance to be mine. That's why I added the lateral-thinking tag.

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  • $\begingroup$ Is the comma in "2,005" used as a decimal separator or a digit group separator? $\endgroup$
    – Wolff
    Commented May 17, 2020 at 14:57
  • $\begingroup$ @Wolff - The OP is a "French Operations Research PhD student" so it is most likely a decimal separator, but I'm not 100% sure. $\endgroup$
    – merrybot
    Commented May 17, 2020 at 16:17

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Based on

The fact that the comma , is sometimes used as a decimal separator, then 2,005 billion = 2,005,000,000 people. According to Wikipedia the US Census Bureau estimates the world population hit 2 billion around 1927, and so it likely hit 2,005 billion a few years later (1930s-ish?). This would make your grandpa between 80-90 years old, which is reasonable!

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  • $\begingroup$ The fact that we use comma as a decimal separator in my country threw me off. I couldn't see the "error". 😀 $\endgroup$
    – Wolff
    Commented May 18, 2020 at 21:24
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A quite unlikely answer:

His grandpa was born in Guatemala in 1807

Because:

If we assume that by 'no more than a billion' he meant about a billion, we know that it is about 1802-1820. If he knew that his country was one 2005th of the world, the we know that his country contained about 500,000 people. The most fitting >!one seems to be Guatemala but it could have easily been any of the following:

Oman, Parma, Holstein, Timor, Austrian Silesia, Dalmatia, Duchy of Carinthia, Duchy of Carniola, Cuba, Nassau, Duchy of Mecklenburg-Schwerin, Canada, British Carribean

There is the problem that:

Grandpa would be between 200 and 220

However:

The question never says when Grandpa is saying this and it could be any time between then and now

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