Probability question 3

Question 3

A person has two children. One is a boy born at an exact specified instant in time (consider a small time interval that tends to zero). What is the probability that the other child is also a boy.

Answer 3

Let us assume that the probability of a boy being born is equal to the probability of a girl being born (both 50%) and also assume that the births of the two children were not simultaneous. The only possibilities are:

  • Another boy was born before the boy born at the exact specified instant time.
  • Another boy was born after the boy born at the exact specified instant time.
  • A girl was born before the boy born at the exact specified instant time.
  • A girl was born after the boy born at the exact specified instant time.

These four possibilities are all equally probable. In two of them the other child is a boy and in two of them the other child is a girl, hence the probability of the other child being a boy is 50%.

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