You're in a hospital where your son was just born. As a nurse wheels your newborn into the nursery, she remarks that yours is the only boy in the room, and the rest of the babies are girls. Once in the nursery, boys are swaddled in blue blankets and girls are wrapped in pink.
A few minutes later, another baby is brought into the nursery and the baby's father, Tom, introduces himself to you. You couldn't see if his child was a boy or a girl, and before you get a chance to ask him, Tom has gone down the hall.
A few minutes later a baby, swaddled in blue, is brought out of the nursery.
What is the probability that Tom's newborn child is a boy?
I always have to go back to reread conditional probability for these problems because it is so non-intuitive to me. It is hard to visualize.
But suppose at the beginning there are no girls, just one boy in the nursery. Then one child is added.
So: if Tom had a boy, the nursery contains: BOY BOY
But if Tom had a girl, the nursery contains: BOY GIRL
Circle one "BOY" word, two of three times,it comes from row one.
If desired, repeat with the initial condition being one boy and one girl plus Tom's baby:
BOY BOY GIRL
BOY GIRL GIRL
still 2 out of 3
Posted by Larry
on 2014-05-10 15:44:14