# Trinagular birthday probabilities

“What is the chance that two people in a group of, say, 30 people would have their birthday on the same day?” I asked Simon as we were sitting on a bench by the river Schelde late last night, waiting for his Dad and sister to arrive by boat. The reason for this question was that one of the professors at Simon’s MathsJam club turned out to have celebrated his birthday exactly on the same day as I the week before. Besides I was afraid of Simon getting bored just sitting there, “enjoying the warm evening”. At first, I thought he didn’t hear my question and repeated myself a couple of times. Then I noticed he was so silent simply because he was completely immersed in the birthday problem.

Eventually, at that time already on Antwerp’s central square, Simon screamed with joy as he told me the formula he came up with involved triangle numbers! “It’s one minus 364/365 to the power of the 29th triangle number!” he shouted. “It’s a binomial coefficient, the choose function!”

## 3 thoughts on “Trinagular birthday probabilities”

1. Yara Marusyk

wow! amazing calculation! Can you imagine that in 2015 we were a group of 6 people – PhD students sharing an office for one year at the University of Groningen, and three of us (including myself) had a birthday on the same day (June 28)?
What would be a probability of that? Would Simon be interested in calculating it? I would love to know and would be happy to share it with my colleagues. 🙂

Liked by 1 person

2. Yara Marusyk

Oh, yes, below I see my answer, you already posted – to exactly our situation! Unbelievable!

Liked by 1 person

• Thank you, Yara! Your comment has prompted Simon to rethink the formula and he came up with a simpler version. (See the updated post).

Like