The probability of being born on a Friday the 13th is 1/214.
Assume the days of people's birth are independent of each other.
(a) You meet two new friends. What is the probability that
both new friends were born on a Friday the 13th?
I got this one, just use the multiplication rule so: . Right!
(b) You meet 31 new friends; what is the probability that
at least one of these new friends was born on a Friday the 13th?
The opposite of "at least one" is "none."
The probability that none were borh on Friday the 13th is: .