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.

  • $\begingroup$ Is the comma in "2,005" used as a decimal separator or a digit group separator? $\endgroup$
    – Wolff
    May 17 '20 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
    May 17 '20 at 16:17

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!

  • $\begingroup$ The fact that we use comma as a decimal separator in my country threw me off. I couldn't see the "error". 😀 $\endgroup$
    – Wolff
    May 18 '20 at 21:24

A quite unlikely answer:

His grandpa was born in Guatemala in 1807


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


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


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.