After the positive response to the recent brain teaser on medical tests, I thought I’d recycle another statistical problem from my university days. This one kept most of us guessing for a while, but once the lecturer pointed us in the right direction most of us got there in the end.

A university course is promoted across different disciplines, with up to 400 students being scheduled to attend the lecture that is held every Tuesday morning. Because the university is very strict on entry criteria, each of the 400 students in the year was born between 1 September 1992 and 31 August 1993.

Assuming the distribution of birthdays across the year is uniform, how many people need to attend the lecture before the probability of two of them being born on the same day exceeds 50%?

Please put your answers in the comments box below and I will come back with the answer next week.

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