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So someone asked me the following riddle:

If a girl is born in the 19th century, and on her first birthday she is 8. How is this possible? I think it's a simple mathematics problem but I can't find a way to figure it out.

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closed as off topic by Arturo Magidin, Henning Makholm, t.b., Asaf Karagila, Jonas Teuwen Nov 21 '11 at 10:32

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Presumably something to do with February 29? –  Zev Chonoles Nov 21 '11 at 2:41
    
@Zev: Feb 29, 1896, to be precise. –  cardinal Nov 21 '11 at 2:42
    
(This is not a mathematics question, just one involving knowing when leap years occur.) –  cardinal Nov 21 '11 at 2:44

1 Answer 1

up vote 4 down vote accepted

If she was born on February 29, 1896, then the next time February 29 came around was in 1904 (due to the 100-year rule of the Gregorian Calendar, 1900 was not a leap year). That is, on her first "birthday", she was 8 years old.

(Cue Pirates of Penzance)

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Sweet nice - I wasn't aware of that rule. –  eWizardII Nov 21 '11 at 2:42
    
@eWizardII: Under the Gregorian Calendar, years divisible by 4 are leap years, unless they are multiples of 100, unless they are multiples of 400; which is why 1700, 1800, and 1900 were not leap years, but 2000 was; 2100, 2200, and 2300 will not be leap years, but 2400 will. –  Arturo Magidin Nov 21 '11 at 2:43
    
+1 for the Pirates of Penzance reference! –  Dilip Sarwate Nov 21 '11 at 3:12

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