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?

(In reply to answer by Ady TZIDON)

Even without seeing any baby chosen from the group, there's a 1/2 probability that Tom's baby is a boy. The fact that a randomly chosen baby was a boy can only increase the probability, not decrease it.

Posted by Charlie on 2014-05-08 10:57:43