My mother’s birthday was the 15th of October. My good friend, the late Michael Jacobs, a fine author, was also born on that day, but many years later. Michael’s mother, the late Maria-Grazia, was born on the 8th of May. I was born on the same day, but many years later.
The chance of two people sharing the same birthday is quite small. It is 1 in 365 or 0.0028. Things get more interesting when one considers a group of people. In a room of 23 people, the chances that two people share a birthday is 0.5 or 50%, and when there are 75 people the probability increases to 0.99 or 99% (see: https://betterexplained.com/articles/understanding-the-birthday-paradox/ and https://en.wikipedia.org/wiki/Birthday_problem).
I have no idea how to calculate the probability that my mother and my good friend share the same birthday AS WELL AS his mother and me sharing the same birthday. It is too long since I studied statistics and probability at school!Even then, I doubt I could have worked it out.