Witches at a Coffee Shop


I got this problem from Rustan Leino, who got it from Alex Pintilie.

I solved it and wrote up my solution.


Each of two witches makes a nightly visit to an all-night coffee shop. Each arrives at a random time between 0:00 and 1:00. Each one of them stays for exactly 15 minutes. On any given night, what's the probability that the witches will meet at the coffee shop?

Solution     Reveal