How many people make the chance of a shared birthday exceed 50 %?
Not quite.
Just 23. At that point the chance is 50.7 %; with 50 people it is already 97 %. This assumes 365 equally likely days.
It's about pairs, not about you
Intuition asks: does anyone share my birthday? For a 50 % chance of that, you really would need 253 other people. But the question is whether any pair matches. And 23 people already form 253 pairs – each one a separate chance of a match.
Where the paradox bites
The same math applies to digital checksums known as hashes. Finding any two texts with the same checksum is much easier than finding a second text that matches one given text. Attacks that exploit this are called birthday attacks.
Fits with this
Nº 063 · Numbers · 50 sec
Why do so many numbers start with a 1?
In real-world data, a 1 comes first more than six times as often as a 9. Tax investigators use that.
Nº 062 · Numbers · 50 sec
A sheet of paper all the way to the Moon?
A sheet is 0.1 mm thick, the Moon about 384,400 km away. The answer sounds like a miscalculation.
