[Guest]

my birthday is in couple weeks so i thought i'd post this little puzzle.

if there are 10 billion people in the world, how many have their birthday on the same month and day as you?

what if i added year?

[Guest]

In 4 years, there are (365 + 365 + 365 + 366) days = 1461 days

So, considering random normal distribution of people being born, 4 in 1461 people would have the same birth date. For someone born on 29th Feb, it would be 1 in 1461.

So, the number of people sharing the same birth date (except 29th Feb) in a population 10 billion is

10 billion x 4/1461 = 27, 378, 508 people.

For people born on 29th Feb, there are 6,844,626 people.

Considering the average global age of 60 years, the 27 million people for each date would be divided in 60.

So, considering same year, the number of people would be = 27,378,508 / 60 = 456,308

For 29th Feb, and same year, it would be = 6,844,626 / 60 = 114,077

Edited June 27, 2009 by DeeGee