The number of days of not leap years is 365.
So, the number of different variations of birthdays for 4 people is 365 * 365 * 365 * 365 = (365)^4
The probability that all of them be one specific day is 365 / (365)^4 = 1 / (365)^3 ≈ 0.00000002.
Answer: 0.00000002
