Birthdays---Challenging

Hi, there. I'm happy to provide a solution to this.

In a 30 day month, the following dates will always land on the same weekday:

Group A: 1-8-15-22-29
Group B: 2-9-16-23-30
Group C: 3-10-17-24
Group D: 4-11-18-25
Group E: 5-12-19-26
Group F: 6-13-20-27
Group G: 7-14-21-28

The thirty days break down into two groups of 5 days, and five groups of four days.

P(birthday on same day)
= P((A and A) or (B and B) or (C and C) or (D and D) or (E and E) or (F and F) or (G and G))
= (5/30)*(5/30) + (5/30)*(5/30) + (4/30)*(4/30) + (4/30)*(4/30) + (4/30)*(4/30) + (4/30)*(4/30) + (4/30)*(4/30)
= (25 + 25 + 16 + 16 + 16 + 16 + 16)/900
= 130/900 = 13/90

That's the probability of the two people having their birthdays on the same weekday in a 30-day month.

Please let me know if anyone reading this has any questions.

Mike