Hello, there!
There are 365 days in a year.
We already know that one person has a birthday on blank day. We will call this person a. That means that there is a 100% chance that they have that birthday.
Because there are 365 days in a year, the odds of person b having a birthday on a certain day is 1/365
Odds of person A having that birthday: 365/365
Odds of person B having that birthday: 1/365
To find the odds of them having the same birthday, we must multiply these odds.
365/365 * 1/365 = 1/365
1/365 = 0.27%
This means that there is a 0.27% chance that both people share the same birthday.
I hope I helped!
Let me know if you need anything else!
~ Zoe